The term 'THE M IN MIB' is an acronym for the MEN in Black, a secret organization that is featured in the 1997 science fiction comedy film of the same name. The three letter solution to this crossword is MEN, which stands for the MEN in Black. The MEN in Black are a group of extraterrestrial law enforcement agents who monitor and police alien activity on Earth. They are known for their black suits, sunglasses, and mysterious demeanor.