Definitions for fifth columnist
Random House Webster's College Dictionary
a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
(originally) Franco sympathizers in Madrid during the Spanish Civil War: the insurgents had four columns marching on Madrid and a “fifth column” of supporters already in the city.
fifth columnist, saboteur(noun)
a member of a clandestine subversive organization who tries to help a potential invader
someone who belongs to a group that secretly undermines another organization
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