WordReference Random House Unabridged Dictionary of American English © 2024fifth′ col′umn, - Governmenta group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
- Government(originally) Franco sympathizers in Madrid during the Spanish Civil War: so called in allusion to a statement in 1936 that the insurgents had four columns marching on Madrid and a fifth column of sympathizers in the city ready to rise and betray it.
fifth′ col′umnist.
Collins Concise English Dictionary © HarperCollins Publishers::
fifth column n - (originally) a group of Falangist sympathizers in Madrid during the Spanish Civil War who were prepared to join the four columns of insurgents marching on the city
- any group of hostile or subversive infiltrators; an enemy in one's midst
fifth columnist n