"to hit," 1931, probably of imitative origin; it is attested by 1975 in the sense of "have sexual intercourse with." Related: Bonked; bonking. As a noun from 1938; in the sexual sense by 1984.
Entries linking to bonk
boink
"have sex with" (v.); "the sex act" (n.), slang by c. 2000, perhaps a bouncier form of bonk in its popular venery sense. Related: Boinked; boinking.
bonkers adj.
"crazy," 1957, British slang, perhaps from earlier naval slang meaning "slightly drunk" (1948), from notion of a thump ("bonk") on the head.