Green’s Dictionary of Slang

1. (US) a noisy talker, a braggart; one who talks too much.

1950		N.Y. Times 10 Aug. 4: Some of the gabbiest flap-jaws in recent history [HDAS].
1960, 1975		Wentworth & Flexner DAS.
1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 15: flap jaw – An over talkative person.
1975		L. Dills CB Slanguage.
2006		Daily Candy 17 Jan. 🌐 You know how it is with secrets: You can’t keep them. (Hey, they don’t call you Flapjaw Magraw for nothing.).

2. verbosity; also attrib.

1999		Rev. of Arts, Literature, Philosophy and the Humanities XVII:3 🌐 In someone else’s hands, this would be just another a flapjaw story – a book concocted by an agent to sell off to a conglomerate publisher.
2005		Seattle Weekly 26 Jan. 🌐 A deceptively disciplined comic performance that would have a lesser actor careening into Monty Python-esque flapjaw.
2005		B. Woodward (ref. to 1972) Secret Man 59: On Saturday, June 24, 1972—just a week after the break-in—[Acting FBI Director] Gray called in the special agent in charge of the Washington field office and 26 agents working the Watergate investigation, and accused them of suffering from ‘flapjaw’.