Quisling was an epithet roughly meaning "a traitor who collaborates with the enemy". The Third Doctor once called the Controller a quisling. When he discovered that the Controller was the third Controller in his family line, the Doctor said that the Controller was from a "family of quislings". (TV: Day of the Daleks)
Behind the scenes
In the real world, "Quisling" is derived from the Norwegian politician, Vidkun Quisling, who ruled Norway for the Germans during World War II.