blow somebody's cover

blow (one's) cover

To expose one's true identity or motives after they had been intentionally concealed (i.e. after one had been "undercover"). The spy fled when he realized that someone had blown his cover. I totally blew my cover when I stupidly wore my school jacket to steal the other team's mascot.
See also: blow, cover

blow somebody’s ˈcover

(informal) discover or reveal the real identity of somebody, especially of a spy, etc: She had been posing as a diplomat, but her cover was blown when she was found sending coded messages to agents.
See also: blow, cover