Why is a traitor called a "turncoat"?
In allusion to a literal turning of the coat inside out that was the custom in the days when feudal lords maintained their own armies. Each lord had a special livery for his servants and swordsmen and when a man left one lord for the service of another, he turned his livery coat inside out so that he might not be mistaken for an enemy upon approaching the castle of his new master. (Why Do We Say It? pg 246)