fifth column
noun /ˌfɪfθ ˈkɒləm/
/ˌfɪfθ ˈkɑːləm/
- a group of people working secretly to help the enemy of the country or organization they are inWord 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.