Flap one’s gums is an idiom. We will examine the meaning of the common idiom flap one’s gums, where it came from, and some examples of its idiomatic usage in sentences.
Flap one’s gums means to babble, to talk incessantly, to say a lot of words that have very little meaning. The expression flap one’s gums came into use in the United States in the mid-20th century and references a constantly moving mouth. Related phrases are flaps one’s bums, flapped one’s gums, flapping one’s gums.
Don’t flap your gums about partying with hot girls in the Hamptons if you’ve never even driven through Long Island. (New York Post)
Most of you flap your gums about other people but you’re still stuck in your seats. (Christian Post)
He flapped his gums all the time about being proud to be different, but in his heart of hearts, ooh, that man is cocky. (Entertainment WEekly)