"crazy," 1957, British slang, perhaps from earlier naval slang meaning "slightly drunk" (1948), from notion of a thump ("bonk") on the head.
Entries linking to bonkers
bonk v.
"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.