to reveal sth one intended to conceal through a slip of the tongue
to blow one's cover
Definitions (Wiktionary)
verb, colloquial, verb-object
to let something slip; to inadvertently reveal something one intended to conceal; to be exposed (syn.) 穿幫, 穿崩, 穿煲, 漏黃, 破空, 露餡子, 露餡, 露餡兒