Определение fifth columnist в Английский Язык Английский Язык словарь
a member of a clandestine subversive organization who tries to help a potential invader
A fifth columnist is someone who secretly supports and helps the enemies of the country or organization they are in. = traitor
person willing to cooperate with an aggressor against his own country, secret supporter of an enemy that engages in espionage or sabotage against his own country