n. 1. Someone who switches to an opposing side; traitor.
Comes from the coats historically worn by soldiers or members of parties, displaying a particular color to identify the group to which the wearer belongs. A traitor or spy might turn their coat inside out, either to hide the color or to change colors so as to appear to belong to the other side.
Usually used figuratively, for anyone who changes sides or otherwise betrays the group to which he or she was allied.
Q: What do you call a treacherous monk?
A: Benedictine Arnold.