"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
mod. loony; silly and giddy. : You are about the wackiest guy I ever met.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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