1936, from Gen. Emilio Mola's comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his "fifth column" (quinta columna) in the city.
Others are reading
Share fifth column
Advertisement
Definitions of fifth column from WordNet
fifth column (n.)
a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst;