Wacky

google

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mid 19th century (originally dialect): from the noun whack + -y1.

wiktionary

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From whack, meaning someone who whacked their head onto something often, hence being weird.

wacky ( countable and uncountable, plural wackies)

etymonline

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wacky (adj.)

"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.