blow someone'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
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

blow someone's cover

INFORMAL
If you blow someone's cover you tell other people who they are or what they are doing, when they are trying to keep it secret. Asking those kind of questions could blow my cover. The young man looked embarrassed, as if he were a spy whose cover had been blown.
See also: blow, cover
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

blow someone's cover

discover or expose someone's real identity.
See also: blow, cover
Farlex Partner Idioms Dictionary © Farlex 2017

blow someone’s cover

tv. to reveal someone’s true identity; to ruin someone’s scheme for concealment. The dog recognized me and blew my cover.
See also: blow, cover
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.
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References in periodicals archive ?
It is not polite to blow someone's cover, especially when that someone is a lady, nay, a veritable grande dame.