fifth column

• POS: Noun Phrase
• HYPHEN: fifth col=umn

CIDE DICTIONARY

fifth column, n. [from a statement during the Spanish Civil War (1936) that the Falange had four columns of soldiers marching on the city, and a fifth column “already there” (i.e. sympathizers inside the Republican lines).].

•  a group of persons inside the battle lines of a territory engaged in a conflict, who secretly sympathize with the enemy, and who engage in espionage or sabotage; -- sometimes also referred to as a trojan horse.  [RP]
•  Hence, any faction of persons within a group who secretly sympathize with an enemy, especially those who engage in activities harmful to the group; an enemy in one's midst; a group of traitors.  [RP]

THESAURUS

fifth column

Judas-like, Trojan horse, Trojan-horse, betraying, collaborationist, collaborator, crypto, double-crossing, fellow traveler, fifth columnist, fraternizer, quisling, quislingistic, saboteur, security risk, subversive, traitorous, treasonable, treasonous, turncoat, two-timing, underground