Speak indiscreetly; also, brag or boast. For example, Now don't go shooting off your mouth about it; it's supposed to be a surprise, or Terry is always shooting off his mouth about how many languages he speaks. [Slang; mid-1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.