The precise origin of 'wacky' is obscure, but it emerged in American slang around the late 19th century. One strong theory connects it to the English word 'whack,' meaning a sharp blow, suggesting that someone who is wacky is behaving as if they have been hit on the head. An alternative theory links it to the Scottish term 'wack,' a colloquialism for a foolish or eccentric person. Regardless of its specific source, the word's playful, reduplicative sound perfectly captures its meaning, evolving from a slang term for 'crazy' to a more lighthearted descriptor for amusingly eccentric behavior.

The word 'wacky' is considered a simplex form. It likely originated as a fanciful, slang formation in American English, possibly influenced by the older word 'whack' meaning 'to strike' or the Scottish 'wack' meaning 'a fool'.