The term "turncoat" is a vivid metaphorical compound from the early 16th century. It originates from the literal idea of a person turning their coat inside out to hide the colors or insignia of one allegiance and reveal those of another, a practical act of betrayal or switching sides. The word "turn" carries its core sense of changing direction, while "coat" provides the tangible object symbolizing one's outward loyalty. Over time, the specific physical action faded, but the powerful image remained, solidifying in English to describe any traitor or renegade who abandons their principles or party.

Structure: turn + coat - turn (verb): From Old English turnian, tyrnan (to turn, rotate), from Latin tornare (to turn on a lathe). In this compound, it functions with the sense of "to change direction or allegiance." - coat (noun): From Old French cote (a kind of garment), of Germanic origin. In this compound, it refers literally to an outer garment.