(originally) a group of Falangist sympathizers in Madrid during the Spanish Civil War who were prepared to join the four columns of insurgents marching on the city
2.
any group of hostile or subversive infiltrators; an enemy in one's midst
Derived forms
fifth columnist
noun
fifth column in American English
a group of people who aid the enemy from within their own country
Derived forms
fifth columnist
Word origin
transl. of Sp quinta columna: first used (1936) by the Sp Nationalist General Mola, who, besieging Madrid withfour columns from the outside, boasted of having a “fifth column” of civilian supporterswithin