Green’s Dictionary of Slang

1. (orig. US) the social elite, the aristocracy; thus, someone who puts on airs.

1919		Sociometry Monographs 36 158: [The] fancy pants from marketing with their ivy league suits and shiny brief cases act like they are running the whole damn business.
1919		Misc. Pamphlets on Peace and Disarmament n.p.: The goodie-goodies and the fancy-pants - the brains minus the brawn - are most apt to fall by the wayside when war comes.
1935		Peters & Sklar Stevedore I iii: All right, fancy pants. You stay put on yo’ tail .
1940		letter in Life 22 Apr. 4/3: You syndicated fancy-pants wouldn’t want we, the people to know of a man who built one of the best networks of roads [...] in the U.S.A.
1957		Kerouac letter 11 Nov. in Charters II (1999) 84: They ain’t nothin compared to Holmes, who knows how to write a virile sentence ... and who has a big heart and doesnt pose as some cool, careless, agnostic-type fancypants.
1965		R. Gary Ski Bum 113: The poodle was sure getting a lot of attention. Fancy pants, he thought.
2000		C. Cook Robbers (2001) 191: I’m Johnny Ray Matthews, and I won’t sit around with my thumb up my ass watching you and Miss Fancy Pants ride a bicycle made for two.
2005		www.lanceandeskimo.com 🌐 Ah, the call of the fancypants! Who hasn’t passed a [...] co-worker who wears a cambric neckerchief and flourishes a gold-headed cane, and [...] said, ‘I wish I were that guy’?

2. an overdressed man, erring towards the effeminate in this preoccupation.

1941		J. Archibald ‘Dog Collared’ in Popular Detective Oct. 🌐 Look, Fancy Pants. Why don’t you do something around here to earn that soup and fish I bought for you, huh?
1949–51		in M. Daly Profile of Youth 73: He’s considered a fancy pants, a mother’s boy or maybe even worse.
1950		ref. in H. Rawson Dict. of Invective (1991) 141: An effete, overdressed man, as in Fancy Pants (film 1950).
1967		J.R. Ramp Consenting Adult 4: ‘You’re not pushing some fancy-pants around in a gay bar’ [Simes:DLSS].
1980		Maledicta IV:2 (Winter) 225: Nancy was reinforced by being a female forename and by the rhyme with fancy (fancy man, fancy pants = sissy).
2000		D.L. Noble Lifeboat Sailors 35: He was called ‘Fancy Pants,’ because he always wore his uniform and was straightlaced.