Definition: A traitor who turns against his or her own country to serve an invader.
[@more@]Usage: This is a relatively new word so far without lexical offspring. The adjective would be "quislingly" which sounds odd. Better use the compound "quisling-like." It sounds queer as a verb, too, though its meaning lends itself readily to verbalization in the sense of "betray to an invader."
Suggested Usage: For those of us who remember World War II, today's word is a powerful condemnation in its literal sense, "The French partisans were always at risk of betrayal by quislings in their area." But as time scrapes on, "quisling" will no doubt take on a more general, diluted sense, "I knew the issue was dead when the new manager and his gang of quislings entered the conference room."