Wiktionary
loose lip
n. (context idiomatic often pluralized English) The practice or characteristic of being overly talkative, especially with respect to inadvertently revealing information which is private or confidential.
Usage examples of "loose lip".
He ran one finger under his loose lip as if he expected to find a tadpole or a small fish, but all he found was mud.
Next man in was no half-wit, but a nimble little ferret of a ship's boy with a loose lip and a cast in one eye.
The Chief chewed his loose lip for awhile, finally and painfully reached a decision.