The term 'whacked-out' is a vivid 20th-century American slang formation. It builds upon the verb 'whack', an onomatopoeic word for a sharp blow, which figuratively came to mean to defeat or exhaust someone. The addition of the adverbial particle 'out' intensifies this meaning to a state of being completely finished or depleted. This literal sense of being utterly exhausted then extended metaphorically to describe mental states perceived as being beyond normal function, such as irrationality or drug-induced disorientation. The compound's logic perfectly captures the journey from a physical impact to a state of total depletion, whether of energy or sanity.

Structure: whack (hit) + out (completely) - whack: Likely of imitative origin, suggesting the sound of a sharp blow. In this compound, it functions as a verb root meaning to hit or strike. - out: From Old English 'ūt'. In this compound, it functions as an adverbial particle indicating a state of being completely affected, exhausted, or beyond normal limits.