A weasel word (also, anonymous authority) is an informal term for words and phrases aimed at creating an impression that a specific and/or meaningful statement has been made, when in fact only a vague or ambiguous claim has been communicated, enabling the specific meaning to be denied if the statement is challenged. A more formal term is equivocation.
The use of weasel words to avoid making an outright assertion is a synonym to tergiversate. Weasel words can imply meaning far beyond the claim actually being made. Some weasel words may also have the effect of softening the force of a potentially loaded or otherwise controversial statement through some form of understatement, for example using detensifiers such as "somewhat" or "in most respects".
Weasel words are likely to be used in advertising and in political statements, where encouraging the audience to develop a misleading impression of what was said can lead to advantages, at least in the short term (in the longer term, systematic deception is likely to be identified, with a loss of trust in the speaker).
The expression weasel word apparently derives from the egg-eating habits of weasels. An article published by the Buffalo News attributes the origin of the term to William Shakespeare's plays Henry V and As You Like It, in which the author includes similes of weasels sucking eggs. The article also claims that this is a misnomer, because weasels do not have a mandible suitable for sucking eggs or blood. Regardless of whether weasels do in fact suck eggs, a belief that they do implies an egg shell devoid of its contents. Thus, words or claims that turn out to be empty upon analysis are known as "weasel words".
Alternatively, definitions of the word 'weasel' include: n. a sneaky, untrustworthy, or insincere person; v. to manipulate shiftily, both of which suggest deception. Finally, the phrase "to weasel out", meaning "to squeeze one's way out of something" or "to evade responsibility". 
The expression is first known to have appeared in Stewart Chaplin's short story "Stained Glass Political Platform" (published in 1900 in The Century Magazine), in which they were referred to as "words that suck the life out of the words next to them, just as a weasel sucks the egg and leaves the shell". Theodore Roosevelt attributed the term to Dave Sewall, claiming that Sewall used the term in a private conversation in 1879. Winston Churchill wrote: "The reserve of modern assertions is sometimes pushed to extremes, in which the fear of being contradicted leads the writer to strip himself of almost all sense and meaning." In another early usage (1916), Theodore Roosevelt argued that "one of our defects as a nation is a tendency to use ...'weasel words'; when one 'weasel word' is used ... after another there is nothing left".
- Numerically vague expressions (e.g. "some people", "experts", "many")
- Use of the passive voice to avoid specifying an authority (e.g. "it is said")
- Adverbs that weaken (e.g. "often", "probably")
Other forms of weasel words include:
- Non sequitur statements
- Use of vague or ambiguous euphemisms (e.g., replacing "firing staff" with "streamlining the workforce")
- Use of grammatical devices such as qualifiers and the subjunctive mood
- Vague generalizations
Generalizing by means of quantifiers, such as many, when quantifiable measures could be provided, obfuscates the point being made, and if done deliberately is an example of "weaseling".
Non sequiturs, i.e. illogical or irrelevant statements can be used e.g. in advertising to make it appear that the statement describes a beneficial feature of a product or service being advertised. Endorsement of products by celebrities, regardless of whether they have ny expertise relating to the product, is an example. I.e. it does not follow (non sequitur) that the endorsement provides any guarantee of quality or suitability.
Spurious authority: use of the passive voice without specifying an actor or agent (e.g. saying "it has been decided" without stating by whom), and citation of unidentified "authorities" or "experts" provide further scope for weaseling. It can be used in combination with the reverse approach of discrediting a contrary viewpoint by glossing it as "claimed" or "alleged". This embraces what is termed a "semantic cop-out", represented by the term allegedly. This implies an absence of ownership of opinion, which casts a limited doubt on the opinion being articulated. The construction "mistakes were made" enables the speaker to acknowledge error without identifying those responsible.
However the passive voice is legitimately used when the identity of the actor or agent is irrelevant. For example, in the sentence "one hundred votes are required to pass the bill", there is no ambiguity, and in fact the actors (i.e. the members of the voting community) cannot practicably be named even if it were useful to do so. 
The scientific journal article is another example of the legitimate use of the passive voice. For an experimental result to be useful, anyone who runs the experiment should get the same result. That is, the identity of the experimenter should be of low importance. Use of the passive voice focuses attention upon the actions, and not the actor (the author(s) of the article).
The middle voice can be used to create a misleading impression, e.g.:
- "It stands to reason that most people will be better off after the changes."
- "There are great fears that most people will be worse off after the changes."
- "Experience insists that most people will not be better off after the changes."
The first of these also demonstrates spurious authority, in that anyone who disagrees incurs the suspicion of being unreasonable merely by dissenting. Another example from international politics is use of the phrase "the international community" to imply a spurious unanimity.
Euphemism may be used to soften and potentially mislead the audience, e.g. "rightsizing" a workforce normally meaning dismissing employees. Other euphemisms commonly used in this area include "headcount reduction" and "downsizing". Jargon of this kind is used to describe things euphemistically.
Restricting information available to the audience is a technique sometimes used in advertisements. e.g. stating that a product "... is now 20% cheaper!" begs the question, "Cheaper than what?". It might be said that "Four out of five people prefer..." something, but this begs the questions of the size and selection of the sample, and the size of the majority (8% for, 2% against and 90% indifferent would be "four out of five" of those who expressed an opinion).
Articles and books
In Report on Unidentified Flying Objects (1956), U.S. Air Force Captain Edward J. Ruppelt described astronomer Dr. J. Allen Hynek's report on the death of Air Force Pilot Thomas Mantell in pursuit of a UFO as "a masterpiece in the art of 'weasel wording'."
Carl Wrighter discussed weasel words in his best-selling book I Can Sell You Anything (1972).
Australian author Don Watson devoted two volumes (Death Sentence and Watson's Dictionary of Weasel Words) to documenting the increasing use of weasel words in government and corporate language. He maintains a website encouraging people to identify and nominate examples of weasel words.
Scott Adams, the creator of the Dilbert comic strip, talks much about 'weasels' (conniving business people) in one of his books, named accordingly: Dilbert and the Way of the Weasel (2002).
- Microsoft Encarta, "weasel words"
- Merriam-Webster, "Weasel, verb"
- Yonghui Ma (2007), "Language Features of English Advertisement", Asian Social Science, March 2007, p 109
- Jason, Gary (1988) "Hedging as a Fallacy of Language", Informal Logic X.3, Fall 1988
- Theodore Roosevelt Association, Theodore Roosevelt Cyclopedia
- E. Cobham Brewer, Brewer's Dictionary of Phrase and Fable
- Rising, Gerry (15 March 1999). "Weasels". Buffalo News. Retrieved 24 December 2013, Buffalo.edu.
- Merriam-Webster online dictionary def'n: http://www.merriam-webster.com/dictionary/weasel
- "Online Free Dictionary". Retrieved 14 April 2014.
- According to The Macmillan Dictionary of Contemporary Phrase and Fable
- New York Times, Sept 2, 1916, "Origin of 'Weasel Words'"
- Crystal, Hilary; David Crystal (2000). Words on Words: Quotations about Language and Languages. University of Chicago Press. ISBN 0-226-12201-8. p. 199
- "Stop him before he votes". "suggests that today's 18-year-olds are too immature to vote. We should be talking about raising the voting age, not lowering it..."
- Viola Ganter and Michael Strube (2009), "Finding Hedges by Chasing Weasels: Hedge Detection Using Wikipedia Tags and Shallow Linguistic Features", Proceedings of the ACL-IJCNLP 2009 Conference Short Papers, page 175
- Garber, Marjorie B. Academic Instincts. Princeton University Press. ISBN 0-691-11571-0. p. 140 "it is alleged"
- "Passive Voice". The Writing Center. University of North Carolina at Chapel Hill. Retrieved 24 December 2013.
- "The Passive Voice" (PDF). English and Theatre – Grammar Pages. Acadia University.
- "Has Downsizing Gone too Far?". University of North Florida. Jacksonville, Florida, USA. December 1995. Retrieved 2007-10-05.
- Report on Unidentified Flying Objects
- Examples and discussion of weasel words
- Jason, Gary (1988) "Hedging as a Fallacy of Language", Informal Logic X.3, Fall 1988
- Weaselwords http://www.weaselwords.com.au/
- Unsuck It http://unsuck-it.com/browse/