Green’s Dictionary of Slang

1. a braggart, a boaster.

1914		H. Hershfield Abie the Agent 21 Aug. [synd. cartoon strip] Don’t be a puplic school loudmouth, Milton!
1929		E. Dahlberg Bottom Dogs 144: A lousy loudmouth, is what Mawx [sic] called him.
1935		(con. 1920s) J.T. Farrell Judgement Day in Studs Lonigan (1936) 577: The order is not particularly concerned with accepting loud-mouths.
1947		I. Shulman Amboy Dukes 65: If only Kenny were smarter and not such a loud-mouth.
1953		J. Thompson Criminal (1993) 14: That big redfaced loudmouth.
1968		N. Heard Howard Street 109: I didn’t mean for that loudmouth to put it on the radio.
1976		Cab Calloway Of Minnie the Moocher and Me 96: mike best, big-time bookmaker, built like Walter Pidgeon, loudmouth.
1981		J. Ellroy Brown’s Requiem 33: I butted in. Jensen was a loudmouth and this could go on all day.
1991		D. Jarman letter 17 Oct. Smiling in Slow Motion (2000) 58: I suppose I could have set myself up as the loudmouth he suggests I am.
2000		J.J. Connolly Layer Cake 3: People [...] who have to be seen as players, the loud-mouths and braggers.
2010		(con. 1973) C. Stella Johnny Porno 59: He found a way to avoid dealing with the loudmouth or he found another weekend job.

2. a lawyer.

1981

G.V. Higgins Rat on Fire (1982) 17: Every landlord in the city’s gonna be in federal court with his own high-priced loud-mouth.