k.i.s.s.!, excl. — Green’s Dictionary of Slang

k.i.s.s.! excl.

[abbr.; orig. milit. usage, later general, also popular in drug rehabilitation circles]

(US) keep it simple, stupid.

1971		(con. 1960s) J.N. Rowe Five Years to Freedom 120: The old ‘KISS’ formula, ‘Keep it simple, stupid,’ served as my guide.
1988		McGill Dict. of Kiwi Sl. 66/1: KISS! Keep It Simple, Stupid! Army acronym.
2000		N.Y. Times 27 June n.p.: Not surprisingly, phrases generated by the computer age tend to be more sardonic and pejorative. ‘Blamestorming’ refers to group discussions devoted to the assignment of blame; the acronym ‘kiss’ means ‘keep it simple stupid’.