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 :
take the fifth
(in the USA) exercise the right of refusing to answer questions in order to avoid incriminating yourself
The reference in this phrase is to Article V of the ten original amendments(1791) to the Constitution of the United States which states that no person shall be compelled in any criminal case to be a witness against himself.