The CapnAndy Gambit is the idea that any experienced town player or power role will be picked off by the scum early, so if a good player is still alive near the endgame, they are probably scum. While there is sound logic behind this gambit, it proves unreliable in practice and can easily be manipulated for the benefit of a less experienced (yet clever) scum team.
This will most often be seen in the form of justifying a late game vote on a skilled player. It is also sometimes used to dismiss votes on good players late in the game as being just a CapnAndy gambit without anything else to it.
It was named after CapnAndy, who invented the gambit.
1. The Mafia Discussion/Interest Check Thread posted by The Ninth Layer.