Quisling is a noun that refers to one who collaborates with an enemy.

Our word of the day comes from a Norwegian army officer who became known for collaborating with the German occupying forces during World War Two. His name evolved into a synonym of ‘traitor’ even before his death in 1945. Quisling is still used today in this same context. For example: A few of us spotted Donna having conversations with members of the other softball team at the bar. I think we may have a quisling on our hands, guys.