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Meaning of fifth column

fifth' col'umn

Pronunciation: [key]
  1. a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
  2. (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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.