Green’s Dictionary of Slang

semi n.

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

[US]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
199020002010
2020
[US]J. Wambaugh Glitter Dome (1982) 261: Al Mackey felt a magnificant semi beginning to engorge!
[Scot]I. Welsh Filth 14: I’ve got a semi and I’m talking to Toal.
[UK]Roger’s Profanisaurus 3 in Viz 98 Oct. 17: huby n. A dobber a semi-on.
[Scot](con. 1980s) I. Welsh Skagboys 86: Ah’ve goat the tweakins av a semi in ma troosers.
E. Hagelstein ‘Neighbor’s Dog’ in ThugLit Aug. [ebook] My semi was subsiding.
[Ire]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].
[UK]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.
[Scot]G. Armstrong Young Team 6: I got a semi thinkin aboot it.

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

2012
20122013
2014
Chief Keef ‘Savage’ 🎵 T-Roy he got the semi / He’ll send yo ass to heaven.
Grizzy ‘Look Like You’ 🎵 Fuck boy introduce you to my semi.