Green’s Dictionary of Slang

wet bottom n.

In phrases

do a wet bottom (v.) (also get a wet bottom) [the result of male ejaculation and vaginal secretions]

of a woman, to have sexual intercourse.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1980		Maledicta IV:2 (Winter) 182: The tendency [for vaginal fluid to flow] is also remembered in the phrases to [...] get a wet bottom and to do, have or perform a bottom-wetter.