Word Origin & History
"crazy, eccentric," 1935, variant of whacky (n.) "fool," late 1800s British slang, probably ultimately from whack "a blow, stroke," from the notion of being whacked on the head one too many times. Wack "crazy person" is a 1938 back-formation; extended form wacko is recorded from 1977. Wack in slang sense
of "worthless, stupid," is attested from late 1990s.