Green’s Dictionary of Slang

1. a (defence) lawyer; in the UK a solicitor or barrister, in the US an attorney; thus in prison the inmates’ representative [orig. SE].

1846		Bell’s Life in Sydney 10 Oct. 3/3: The worthy coroner [...] appeared as mouthpiece for the defendant.
1857		‘Ducange Anglicus’ Vulgar Tongue.
1865		Leaves from Diary of Celebrated Burglar 56/1: Now he was ‘pinched’ and this raffle was to help get him a ‘mouth-piece’.
1867		Nat. Police Gaz. (NY) 28 Sept. n.p.: Which [statement] Munday declared by a ‘mouthpiece’ was derogatory.
1873		Sl. Dict. 230: Thieves and their associates always speak of a counsel as a mouthpiece.
1883		J. Greenwood Odd People in Odd Places 18: A mouthpiece [...] is another word for a defending counsel .
1890		Sporting Times 22 Feb. 2/2: Then Benzon’s mouthpiece had a go, and spoke for an hour and ten minutes without a break.
1895		C. Crowe Aus. Sl. Dict. 50: Mouth Piece, a lawyer.
1892		Proc. Old Bailey 14 Nov. 112: I asked him what he was there [i.e. on trial] for—he told me ‘For murdering an old toff in the Borough.’ [...] I said, ‘What was he?’—he said, ‘A mouthpiece’—I said, ‘What?’—he said, ‘A solicitor’.
1900		Marvel 6 June 6: Hadn’t he got a mouthpiece at the trial?
1904		Sydney Sportsman (Surry Hills, NSW) 28 Sept. 3/6: ‘Did she stick?’ ‘Blast ’er, no; didn’t even get me a mouthpiece’.
1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 890: A ‘swell mouthpiece’ is a very good lawyer, while a very bad one is called a ‘shyster.’.
1920		C. Sandburg ‘Cahoots’ in Smoke and Steel 45: Go fifty-fifty. / If they nail you call in a mouthpiece. / Fix it, you gazump, you slant-head, fix it.
1923		Northern Whig 12 Sept. 8/6: I was fullied, and a black-box asked a couple of foont and ten deaner to be my mouthpiece.
1931		C.B. Yorke ‘Mob Murder’ in Gangland Stories Mar. 🌐 Our mouthpiece had tried to carry the case to a higher court, but failed.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 7: Mouthpiece:Solicitor.
1936		Queenslander (Brisbane) 2 July 4/4: Then to this Bleareye Beak I cried: / ‘Hold on! What mouthpiece may I have to guide / Me in this, little piece of ‘biz’?’ / But - too late! ‘Ten years’ His Nibs replied!
1946		Mezzrow & Wolfe Really the Blues 268: The judge appointed two more attorneys and these mouthpieces finally made a deal with the D.A.
1950		News (Adelaide) 1 July 5/3: ‘He wants to jack up, but his mouthpiece drums him to nod the nut, and he cops the clock in boob’ .
1950		P. Hoskins No Hiding Place! 191/1: Mouthpiece. Solicitor or counsel to defend the prisoner.
1959		Mad mag. Oct. 46: He’ll kill his mouthpiece when he gets outside.
1960		J.R. Ackerley We Think The World Of You (1971) 17: It seemed that he could have a ‘mouthpiece’ – a poor prisoner’s defence counsel.
1967		K. Tennant Tell Morning This 393: ‘Have a bit of trust in your mouthpiece’.
1977		E. Bunker Animal Factory 15: Fuckin’ mouthpieces are lyin’ bastards.
1984		B. Ellem Doing Time app. C 238: Your chances of favourable verdicts increase with your ability to purchase a mouthpiece.
1989		H. Rawson Dict. of Invective (1991) 14: Members of the bar seem to be in especially bad odor, as indicated by the existence of mouthpiece, petifogger, and shyster.
1999		Guardian G2 28 June 3: Weary of being labelled the ‘mob mouthpiece’.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 119/2: mouthpiece n. 1 a lawyer 2 an inmate chosen as the spokesperson for his fellow inmates.
2003		Bug (Aus.) May 🌐 Wayne was totally pissed that this bloke and his fancy legal mouthpiece had gone running to the United Nations.

2. (US) an informer.

1900		Flynt & Walton Powers That Prey 40: I’m a bad lot if you like, but I wouldn’t turn mouthpiece for the whole five thousand.
1901		J. Flynt World of Graft 125: A mouth-piece is a thief who tells tales to the police about his pals.
1931		Irwin Amer. Tramp and Und. Sl. 131: Mouthpiece. – Formerly, a stool pigeon who spoke to the police about a criminal or gang.
1934		Dundee Courier 23 Aug. 3/4: Notyre wrote [...] the word ‘bogey’ [...] then wrote [...] ‘The geeser talking to the plain clothes copper is a mouthpiece’.

3. (US black) gold caps on one’s front teeth.

2005		www.paulwallworld.com May 🌐 While old-school rappers like Slick Rick and MC Shan kept their mouth-bling simple with a couple of gold caps, today’s grill gods sport mouthpieces costing more than your entire wardrobe.
2008		posting at www.seemygrill.com 🌐 I’m really feelin da mouth piece! im getn my gurl a pair .