The term 'flap one's gums' is an idiom that means to talk incessantly or to chatter aimlessly. The crossword solutions for this term are YAP (3 letters), NATTER (6 letters), and GAB (3 letters). YAP is a verb meaning to talk rapidly and continuously, often about trivial matters. NATTER is a verb meaning to talk idly or casually, especially about unimportant matters. GAB is a verb meaning to talk in an animated or lively manner, especially about unimportant matters. All three words are synonyms for the phrase 'flap one's gums'.