Green’s Dictionary of Slang

1. (orig. US) something or someone disgusting.

1969		Current Sl. IV:1 9: Gross out, n. Anything unbelievable or crude.
1979		S.F. Examiner and Chronicle 1 Apr. A:12: It’s just toooo much (late ’40s). A real drag (’50s), a bad trip (’60s), and, naturally, a gross-out (’70s).
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 20: The kid [...] turns completely into a katydid [...] which was so much of a grossout that Horace almost lost his voice.
2004		Allen & Ishi-Gonzxales Hitchcock: Past and Future 34: Psycho was a watershed film, [...] foreshadowing the I970s trend toward gross-out.
2024		J. Meades Empty Wigs (t/s) 751: [W]hat a gross-out... if the wind had changed I’d have been lumbered with an open mouth for ever.

2. (US campus) a contest between students to see who can be most disgusting.

1966		Current Sl. I:3 3/2: Gross-out, n. A contest or competition at being gross.
1979		D. Gram Foxes (1980) 77: Creature’s having a gross-out and he wanted you there.