Green’s Dictionary of Slang

1. (US Und.) a lawyer.

1992

Bentley & Corbett Prison Sl. 21: Mouthpiece An attorney. (Archaic: cop a plea, two-bit mouthpiece).

2. attrib. use of sense 1.

1950

Goldin et al. DAUL 50/1: Cop-a-plea mouthpiece. An unscrupulous criminal lawyer who collects his retainer fee and, if no further sums are forthcoming, advises client to plead guilty whatever the merits of his case.