Green’s Dictionary of Slang

to perform soixante-neuf (on someone); thus sixty-niner n., a person who performs this act.

1933		Ersine Und. and Prison Sl. 67: sixty-nine, n. A perverted sexual act. sixty-niner, n. A sexual pervert.
1949		Monteleone Criminal Sl. (rev. edn) 210: sixty-niner A sodomite.
1950		Goldin et al. DAUL 195/2: Sixty-niner. One who practices the sixty-nine act.
1967		R. Conot Rivers of Blood 165: Had your rocks blown lately, boy? Come on and we’ll sixty-nine ya!
1972		Hall & Adelman Gentleman of Leisure 156: He said we should ... sixty-nine.
1979		‘Iceberg Slim’ Airtight Willie and Me 10: A match flared [...] illuminating two broads sixty-nining.
1990		S. Morgan Homeboy 82: Conventioneers paid pretty pictures to watch two bimbos sixty-nine on their banquet table.
1997–2002		Alt. Eng. Dict. 🌐 sixty-nine (compound noun) perform oral sex.
1998		‘Bill E. Goodhead’ Nubile Treat 🌐 His tongue was sliding all over her glowing puss and she was using her tongue to send thrill after thrill of pleasure zinging up the hard shaft of his prick. They were sixty-nining away.
2003		K. Cage Gayle 94/2: sixty nine v. perform mutual fellatio between two persons, suck each other’s penises at the same time.
2006		G. Malkani Londonstani (2007) 31: Fuckin useless tongue. Probly couldn’t even sixty-nine it.
2023		(con. 1962) J. Ellroy Enchanters 53: They’re fucking and sucking here. They’re 69’ing there.