A fuddy-duddy (or fuddy duddy or fuddy-dud) is a person who is fussy while old-fashioned, traditionalist, conformist, or conservative, sometimes almost to the point of eccentricity or geekiness. It is a slang term, mildly derogatory but sometimes affectionate too and can be used to describe someone with a zealous focus on order.
"Fuddy-duddy" is considered a word based on duplication and may have originated as a fused phrase made to form a rhyming jingle. Duddy is similar to Daddy and may have caught on from children's rhyming.
Douglas Harper of the Online Etymology Dictionary reports it from "1871, American English, of uncertain origin." However, Dictionary.com Unabridged compares it to a Northern English dialectal term: "1900-05; of obscure origin; compare dial. (Cumberland) duddy-fuddiel a ragged fellow."
Gary Martin states: "William Dickinson's A glossary of words and phrases pertaining to the dialect of Cumberland, 1899, has:
"Duddy fuddiel, a ragged fellow""
and "in 1833, the Scots poet James Ballantyne wrote The Wee Raggit Laddie:
Wee stuffy, stumpy, dumpie laddie,
Thou urchin elfin, bare an' duddy,
Thy plumpit kite an' cheek sae ruddy,
Are fairly baggit,
Although the breekums on thy fuddy,
Are e'en right raggit." 
"Fuddy-duddy" is used to indicate "stuffiness" and "outmoded tastes and manners". For example, the Rolls Royce car manufacturer was referred to as a "fuddy-duddy" brand in a 2004 Popular Science article.
Ambrose Bierce's story Who Drives Oxen Should Himself be Sane, published in 1918, starts out with a use of the word and discussion of it as a "unique adjuration". The term is also used in the title of juvenile fiction including Kay Hoflander's The Chautauqua Kids and the Fuddy Duddy Daddy: A Tale of Pancakes & Baseball, and the Uncle Fuddy-Duddy series by Roy Windham and Polly Rushton.
"Fuddy-duddy" is often used to refer to a man perceived as stodgy or foolish. It has been used throughout the 20th century, but its origins are unknown. The short form "fud" may relate to the Bugs Bunny cartoon character Elmer Fudd. The terms frump and old fart have also been used as words to designate similar qualities.
Female figures have been labeled with terms of a similar meaning, including "school marm", or "marm", which could be used for an older female disciplinarian such as a stereotypical type of strict teacher.
"Fuddy-duddy" "was often used as a verb by a native of the state of Maine... in the sense of 'to act in a foolish or ineffectual manner".
- Tom Dalzell The Routledge Dictionary of Modern American Slang and Unconventional English
- Karen O'Connor Fuddy-duddy Walkin' with God Ain't for Wimps: Spirit-Lifting Stories for the Young at Heart page 67-68
- Anatoly Liberman Word origins—and how we know them: etymology for everyone pages 54, 57, 59
- Harper, Douglas. "fuddy-duddy". Online Etymology Dictionary. Accessed December 1, 2016.
- "the definition of fuddy-duddy". Dictionary.com Unabridged. Random House, Inc. Retrieved December 1, 2016.
- World Book Dictionary (Unabridged). Chicago: World Book, Inc. 2003. p. 860. ISBN 0716602997. Retrieved December 1, 2016.
- Stephany Wilkinson Fuddy-Duddy Brand Spawns Luxury Rocket Man & Machine; Hard Fast Shiny Objects and Why We Love Them, July 2004 120 pages Vol. 265, No. 1 ISSN 0161-7370 Published by Bonnier Corporation, Popular Science page 26
- Ambrose Bierce Can such things be? Publisher Boni & Liveright, 1918 Original from the University of California, Digitized Nov 6, 2008
- The Chautauqua Kids and the Fuddy Duddy Daddy: A Tale of Pancakes & Baseball, Kay Hoflander - Juvenile Fiction - 2007 - 64 pages
- Uncle Fuddy-Duddy Rabbit Tales, Roy Windham, Polly Rushton - Juvenile Fiction - 2004 - 24 pages
- Uncle Fuddy-Duddy Learns to Fly!, Roy Windham, Polly Rushton - Juvenile Fiction - 2005 - 24 pages
- Philip Herbst  Wimmin, wimps & wallflowers: an encyclopaedic dictionary of gender and... page 108
- Dictionary of American Regional English: D - H, Volume 2 By Frederic G. Cassidy page 597