fifth column

noun

Definition of FIFTH COLUMN

: a group of secret sympathizers or supporters of an enemy that engage in espionage or sabotage within defense lines or national borders

— fifth col·um·nism \-ˈkä-ləm-ˌ(n)i-zəm\ noun

— fifth col·um·nist \-(n)ist\ noun

name applied to rebel sympathizers in Madrid in 1936 when four rebel columns were advancing on the city

First Known Use: 1936

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