Green’s Dictionary of Slang

1. (US) to prepare, to get ready.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 51: prep, v. To prepare.
1911		G. Bronson-Howard Enemy to Society 112: ‘Sheeny Mike’ made a statement to them that he had been ‘trailing’ Mrs. Vaughan for days and that he was all ‘prepped’ to turn this particular trick.
1920		Ade Hand-made Fables 242: These many Snows he had done nothing but Prep himself for a sweeping Entrance.
1957		(con. 1944) A. Myrer Big War 415: I’ve got to prep this kid from JASCO right away.
1978		R. Price Ladies’ Man (1985) 179: I [...] jumped into the shower and started to prep.
1982		G.V. Higgins Patriot Game (1985) 36: Can I see Walker now that you’ve prepped for this nitwit in there with him?
1997		Simon & Burns Corner (1998) 76: They gear up, prepping the flesh for the doctor’s grand rounds.
2001		Source Aug. 52: Busta’s personal barber, Bo Butter, preps Flipmode’s head honcho for a photo shoot.
2010		Codella and Bennett Alphaville (2011) 347: Prepping and cooking in catering hall kitchens.
2012		(con. 1980s) I. Welsh Skagboys 271: Ah forgot the power ay this shite. Lesley never prepped up that much but ah collapse back oantae the Royal Stuart.
2017		L. McInerney Blood Miracles : He had half a story prepped before she found him.
2019		C.D. Rosales Word Is Bone [ebook] The men ignored the cockwranglers [and] began prepping the cocks by bringing them just outside of bloodletting range.
2020		C. Hiaasen Squeeze Me 289: ‘We’ll be slammed all day, prepping for the ball’.
2021		P Howard Braywatch 34: ‘Sorcha said you’re prepping for your interview’.

2. (US campus, also prep out) to get dressed up, esp. over-dressed, or elaborately made up.

1982

Eble Campus Sl. Spring 7: prep, prep out, prepped-out, prepped up – wearing an assemblage of certain name-brand clothing and jewelry associated with the preppy look. Also expressing the impression of overkill given by too much makeup.