Smack on the Mouth' is a phrase that is used to describe a physical act of aggression, usually involving a slap or punch to the face. It is considered a form of assault and is illegal in many countries. The crossword solution for this term is 'KISS, ' which is an acronym for 'Keep It Simple, Stupid.' This acronym is often used to remind people to keep their actions and words simple and straightforward, rather than complicated and convoluted.