an organized group of people sympathizing with and working for the enemy within a country at war or otherwise under attack
Fifth column is a translation of the Spanish phrase quinta columna : during the Spanish Civil War, an extra body of supporters was claimed by General Mola as being within Madrid when he besieged the city with four columns of Nationalist forces in 1936.
Related Idioms :
dodge the column
shirk your duty
avoid work - British informal
Column is a military term which refers to the usual formation of troops for marching.