The adjective 'wack' emerged in American slang during the late 20th century. It is widely considered a back-formation from the older term 'wacky', meaning 'eccentric or irrational', which itself may derive from the earlier 'whacky' or from 'whack' meaning 'a blow'. The semantic evolution likely moved from the sense of being 'struck' or 'out of order' to meaning 'foolish' and finally to its dominant modern sense of 'terrible' or 'uncool'. Its adoption and popularization are heavily tied to hip-hop culture and vernacular.

The word 'wack' is a simplex form. It is a slang term of uncertain origin, likely a back-formation or alteration of the earlier term 'wacky'.