Green’s Dictionary of Slang

1. silly, immature [SAmE prep school].

1900

E.H. Babbitt ‘College Words and Phrases’ in DN II:i 51: preppy, adj. Silly, immature.

2. mainstream in style and thought, as a result of having been to a major US prep school [preppie n.].

1951		Yale Daily News 24 Nov. n.p.: There are ‘preppy’ groups and ‘doggy’ groups—never the twain shall meet.
1970		(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 101: Crew cut, flat top, Princeton [...] Tweedy and preppy.
1977		J. Sayles Union Dues (1978) 277: Grant Parke III remembers Green from [...] the Eastern Preppie League.
1978		L.K. Truscott IV Dress Gray (1979) 268: One of them got stood up by some Princeton preppie fuckstick.
1981		Eble Campus Sl. Fall 5: preppie – associated with fraternity and sorority styles.
1985		J. Wambaugh Secrets of Harry Bright (1986) 66: It was full of pictures of her preppy nineteen-year-old daughter.
1991		B. Hamper Rivethead (1992) 82: The place was now nothing more than a preppie playpen for the terminally dull.
2000		Guardian Rev. 25 Feb. 5: Dickie’s awful preppie buddy Freddy Miles.
2000		Indep. on Sun. Travel 30 Jan. 11: If I couldn’t get a kilted lover, I’d try for a preppy American boyfriend instead.
2004		P. Howard PS, I Scored the Bridesmaids 130: I find that preppy college look so eighties.
2010		J. Ellroy Hilliker Curse 20: They wore [...] prepped out-out civvies in the evenings. Madras shirtwaists and tartan kilts [Ibid.] 23: A black beret for preppy beatnik Kay.