fifth col·umn
noun/ˌfɪfθ ˈkɒləm/
/ˌfɪfθ ˈkɑːləm/
- a group of people working secretly to help the enemy of the country or organization they are in
第五纵队（为所在国家或组织的敌人秘密工作的一群人） Word OriginThe term dates from the Spanish Civil War, when General Mola, leading four columns of troops towards Madrid, declared that he had a fifth column inside the city.