"crazy person," 1938, back-formation from wacky. Adjective in slang sense of "worthless, stupid," is attested from late 1990s.
"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.
extended form of wack, by 1971.
updated on July 13, 2012