Green’s Dictionary of Slang

1. a chatterer, a gossip; an obsessive talker.

1955		L.A. Times 3 Nov. 5/1: I took the ‘Are You a Gossip?’ quiz. Man, what a motormouth.
1963		W.C. Anderson Penelope 103: Shut up, motormouth, and listen.
1974		G.V. Higgins Cogan’s Trade (1975) 121: I didn’t know he was motor-mouth [...] and he was.
1981		J. Sullivan ‘The Second Time Around’ Only Fools and Horses [TV script] Listen oi, listen motor-mouth!
1998		Source Nov. 43: Sonny Liston, the brutish heavyweight defeated by charismatic motormouth Cassius Clay.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 211: A posse of henchmen, chief among them a scruffy, cockney motormouth named Johnny the Lip.
2007		L. Redhead Cherry Pie [ebook] ‘Sudafed [...] works like speed, turns me into a motor mouth’.
2011		Sowetan (Johannesburg) 24 June 🌐 Once again motor-mouth Luclay seems to have survived the chop.
2017		Philadelphia Inquirer (PA) 29 Jan. F07/5: Yeah, maybe we were a bit snarky! Even Mrs Motormouth said so.
2022		J. Hannaham Didn’t Nobody Give a Shit 64: His daughter was a filterless motormouth.

2. the mouth of such a person.

1976		P. Conroy Great Santini (1977) 391: He put the emergency brake on that motor mouth of his.
1981		(con. 1966) P. Conroy Lords of Discipline 216: I want to see the ol’ wise-cracking Will back and running his motor-mouth.
1997		G. Sikes 8 Ball Chicks (1998) 56: A pint-sized teenager with a motormouth named Baby Crazy stood up and began writing rollin’ twenties with a Magic Marker on the wall.
2008		Sun. Indep. (S. Afr.) Culture 27 Jan. 11: Exhilarating ride with DC’s slick motor-mouth.