The term "wacko" emerged in American slang during the mid-20th century as a humorous and informal intensification of "wacky." While "wacky" itself has obscure origins, possibly linked to the notion of being figuratively "struck" or "whacked" out of one's senses, "wacko" follows a common pattern in English slang of adding an "-o" suffix to create a more vivid, often derogatory, noun or adjective. This formative process mirrors other slang creations like "weirdo" or "jumbo." The word's journey reflects the creative, playful, and often opaque nature of slang etymology, where sound and rhythm often take precedence over classical roots.

This word is a simplex form, likely a playful alteration of the word "whacky" or "wacky," which itself is of uncertain origin but may be related to the English dialect word "whack" meaning "a blow" or "to strike."