Green’s Dictionary of Slang

1. (US gay) of a place, e.g. a club, half-homo- and half-heterosexual in clientele.

1964

Lavender Lex. n.p.: semi:–Meaning that the place is half gay and half straight. Or that homosexuals are allowed but not highly encouraged. In many places this also means that it is straight in the daytime and gay at night.

2. (also semi-on) a semi-erection of the penis.

1981		J. Wambaugh Glitter Dome (1982) 261: Al Mackey felt a magnificant semi beginning to engorge!
1998		I. Welsh Filth 14: I’ve got a semi and I’m talking to Toal.
1999		Roger’s Profanisaurus 3 in Viz 98 Oct. 17: huby n. A dobber a semi-on.
2012		(con. 1980s) I. Welsh Skagboys 86: Ah’ve goat the tweakins av a semi in ma troosers.
2013		E. Hagelstein ‘Neighbor’s Dog’ in ThugLit Aug. [ebook] My semi was subsiding.
2015		L. McInerney Glorious Heresies 222: [H]ere he was after nine months of celibacy with barely a semi for the lot of them [i.e. pornographic images].
2017		Times Weekend Rev. 8 July 🌐 They have boners. They aren’t embarrassed. They cup, adjust. ‘I’ve got a semi on,’ they might say, conversationally.
2020		G. Armstrong Young Team 6: I got a semi thinkin aboot it.

3. (US black) a semi-automatic weapon.

2012		Chief Keef ‘Savage’ 🎵 T-Roy he got the semi / He’ll send yo ass to heaven.
2014		Grizzy ‘Look Like You’ 🎵 Fuck boy introduce you to my semi.