blow one's cover


Inadvertently give away one's secret identity, as in Mary came to the annual meeting pretending to be a shareholder and hoped no one would blow her cover. This expression uses blow in the sense of “expose or betray,” a usage dating from Shakespeare's day.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.