verb intr.: To betray, especially by collaborating with an enemy.
Back-formation from quisling (traitor), after Norwegian army officer Vidkun Quisling, who collaborated with the German occupying forces during World War II. Earliest documented use: 1940.
“The AK and subordinate units made … 5700 attempts on officers of different police formations, soldiers, and volksdeutschs (Polish citizens of German origin that volunteered to quisle with Germans).”
Polish Contribution to the Allied Victory in Second World War; Business Recorder (Karachi, Pakistan); Jun 11, 2005.