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The phrase four-letter word refers to a set of English-language words written with four letters which are considered profane, including common popular or slang terms for excretory functions, sexual activity and genitalia, and (depending on the listener/reader) sometimes also certain terms relating to Hell and/or damnation when used outside their original religious context(s), and/or slurs. The "four-letter" claim refers to the fact that a large number of English "swear words" are incidentally four-character monosyllables. This euphemism came into use during the first half of the twentieth century.
Older discussion 
Common four-letter words (in this sense) that are widely considered vulgar or offensive to a notable degree include: cunt, fuck (and regional variants such as feck, fick and foak), jism (or gism), jizz, piss, shit, twat and tits. Piss in particular, however, may be used in non-excretory contexts (pissed off, i.e. "angry", in US English; pissed, i.e. "drunk" in UK English) that are often not considered particularly offensive, and the word also occurs several times with its excretory meaning in the King James Bible. Several of these (including even piss, despite its biblical pedigree) have been declared legally indecent under the United States Federal Communications Commission (FCC) TV and radio open-airwave broadcasting regulations.
A number of additional words of this length are upsetting to some, for religious or personal sensitivity reasons, such as: arse (UK), damn, fart, hell, wang, and wank (UK). Racial, and sexual-preference slurs may also qualify, such as mong, gook, kike, spic, coon, dago and dyke. Several "four-letter words" have multiple meanings (some even serving as given names), and usually only offend when used in their vulgar senses, for example: cock, dick, knob, muff, puss, shag (UK) and toss (UK). A borderline category includes words that are euphemistic evasions of "stronger" words, as well as those that happen to be short and have both an expletive sound to some listeners as well as a sexual or excretory meaning (many also have other, non-vulgar meanings): butt (US), crap, darn, dump, heck, mofo (US), poop (US), slag (UK, NZ, AUS), slut and turd, as several examples. Finally, certain four-lettered terms with limited usage can be considered offensive by some, within the regional dialect in which they are used, such as mong and mary.
Occasionally the phrase "four-letter word" is humorously used to describe common words composed of four letters. Typical examples include the word work, implying that work can be unpleasant, or the game of golf, jokingly referred to as a four-letter word when a player's pastime becomes an exercise in frustration. Charlotte Observer journalist Doug Robarchek noted in 1993 how many politicians have names four letters, "Ever notice how many U.S. politicians have names that are also words? Ford, Dole, Duke, Bush, Gore . . . and how many make us think of four-letter words?"
Similar euphemisms in other languages 
- Dutch: A similar tradition occurs with "three-letter words", e.g. kut ("cunt"/"twat"), pik and lul ("cock"/"dick"/"prick").
- French: the word merde ("shit") is sometimes referred to as le mot de cinq lettres ("the five-letter word"). Also, profanities in French are usually called gros mots (big words).
- German: the phrase Setz dich auf deine vier Buchstaben ("sit down on your four letters") is mainly used speaking to children, as it refers to the word Popo, meaning "rump" in baby talk. A variant, Setz dich auf deine fünf Buchstaben ("sit down on your five letters"), alludes to the vulgar use of the word Arsch, meaning "arse" (UK) or "ass" (US).
- Hebrew: another meaning of "four-letter word" (in Greek, tetragrammaton) is the Hebrew name of the Abrahamic God, that is, י-ה-ו-ה (commonly transliterated as "YHWH", "Yahweh", and "Jehovah"), which many practicing Jews do not speak aloud and protect when written (see Geniza). It is an example of the quadriliteral words of Hebrew.
- Latin: a common insult used to be Es vir trium litterarum, meaning "you are a man of three letters". The underlying implication was that the addressed was a fur, meaning "thief", although if challenged, the speaker could always claim he simply meant vir, that is, "man".
- Polish: the word dupa ("arse"/"ass") is called cztery litery ("the four letters"). Historically, also kiep, which formerly used to be a taboo word meaning "female genitals", but presently is a mild or humorous insult meaning "a fool". Also, the word kurwa ("street corner prostitute") is euphemistically called słowo na k ("the word starting with letter k"). There is also a phrase Siadaj na cztery litery (sit down on your four letter), meaning sit on your arse.
- Russian: the word хуй ("cock"/"dick"/"prick"), the most common obscenity, is called "the three-letter word" (russ.: "слово из трёх букв") or just "three letters" (russ.: "три буквы") and is one of the key words of the "Russian mat".
Good authors too who once knew better words,
Now only use four-letter words.
Use in titles 
- Cheap Trick's 1982 album One On One finishes with a song titled "Four Letter Word".
- The fact that love is a word with four letters has been used in several popular songs, including "Love Is Just a Four-Letter Word" written by Bob Dylan and performed by Joan Baez, "Four Letter Word" written by Ricki and Marty Wilde and performed by Kim Wilde, "4 Letter Word" written by Claude Kelly and Matt Squire and performed by David Cook .
- A television show called Love Is a Four-letter Word was produced by ABC in Australia.
- The band Cake made a play on words in their song "Friend Is a Four Letter Word."
- In a song sung by Cilla Black and covered by The Smiths, "Work Is a Four-letter Word," this phrase is used to describe work as obscene.
- Work Is a Four-Letter Word is the title of a 1968 British comedy film.
- That Four-Letter Word is a 2006 independent film from India.
- Welsh punk band Four Letter Word, formed in 1991, named themselves after the phrase.
- Jack Ingram's song "Love You", uses love as a play on another four letter word: "Yeah, I'm sick an' lovin' tired of all your lovin' around".
- Metallica uses the line "Love is a four-letter word" in the song "The Day That Never Comes" from their album Death Magnetic.
- A Four Letter Word is also the title of a 2007 gay-themed movie starring Jesse Archer and Charlie David.
- R&B singer Raheem DeVaughan used the "love" meaning in his song "Four Letter Word" on his 2008 album Love Behind The Melody.
- "Four Letter Word" is also the title of a 2003 song by Def Leppard.
- American punk-rock band Gossip released a track entitled 'Four Letter Word' on their 2009 album Music For Men.
- The 2007 Cold in California album by Ingram Hill includes a track entitled 'Four Letter Word.'
- The opening track of Beady Eye's 2011 album Different Gear, Still Speeding is entitled 'Four Letter Word.'
- Echobelly on their album On (Echobelly album) included a song named: 'Four Letter Word.'
- Love Is a Four Letter Word, 2012, Jason Mraz
The "seven dirty words" 
A famous citing of a litany of four-letter (and some longer) vulgar words was in the US FCC's censorship of comedian George Carlin's radioplay of his comedy routine "The Seven Words You Can't Say on Television", better known as the "Seven dirty words" skit ("shit, piss, fuck, cunt, cocksucker, motherfucker, tits", in that order). Carlin later expanded the list in live and cable-televised performances to include over 100 words and phrases.