Turncoat

A turncoat is someone who changes their mind and supports the opposite team or group.

So imagine two teams are playing a game. Let's call them the Red team and the Blue team. Let's say you are playing for the Red team but then you decide you want to switch teams and play for the Blue team instead. That would make you a turncoat. It's sort of like changing sides in a game or a fight.

Turncoats can be seen as dishonest or untrustworthy because they switched sides, and it can sometimes be confusing for their old teammates and new teammates.