loose lip

plural loose lips

1. idiomaticoften pluralizedThe practice or characteristic of being overly talkative, especially with respect to inadvertently revealing information which is private or confidential.

   Loose lips might sink ships.

   c. 1940, Slogan used by US government during World War II

   2 more examples

   In the classic dramas of private investigation, the cheeky quip is the tough guy's challenge to toughness. In Fletch the quick, smartly paced gags somehow read as signs of vulnerability. . . . Every minute you expect the hero's loose lip to be turned into a fat one.

   1985 June 3, Richard Schickel, “Cinema: Gliberated in Dreamland Fletch”, in Time

   Poor fellow, he had "cemented my reputation forever as a guy who tells too much truth." . . . But his loose lip has ultimately worked out for him.

   1996 August 25, Donald E. Westlake, “Tough Guys Don't Shut Up”, in New York Times, retrieved 29 May 2011

   Synonyms: big mouth, loquaciousness