fifth column

Meanings

1. A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.
2. Used other than figuratively or idiomatically: see fifth, column.

Etymology

From a 1936 radio address by Spanish Nationalist general Emilio Mola, in which he spoke of four of his army columns moving on Madrid and a fifth column (quinta columna) consisting of his militant supporters within the capital, intent on undermining the Republican government from within.