Other forms: smoking guns
A smoking gun is a clear piece of evidence. A babysitter who's trying to figure out which kid ate the last chocolate cupcake might consider one child's frosting-smeared face to be a smoking gun.
Evidence that's so strong it seems undeniable is a smoking gun, whether it's a shredded piece of math homework in your dog's mouth or a clear fingerprint on a stolen diamond ring. A smoking gun almost always proves clearly that a particular person is guilty of something. It comes from the idea that a recently fired gun — still smoking — in a suspect's hand proves his guilt, and it was first used in the 1970s.