A fifth column is a group of people who clandestinely undermine a larger group such as a nation from within. A fifth column can be a group of secret sympathizers of an enemy that are involved in sabotage within military defense lines, or a country's borders. A key tactic of the fifth column is the secret introduction of supporters into the whole fabric of the entity under attack.
Bobby: Not ironically, Fifth Columnist rhymes with Filthy communist.
Joe: You got me there, Bobby, I have no idea.
Bobby: Yeah, right.