Green’s Dictionary of Slang

verbose, chattering.

1979		N.Y. Amsterdam News 20 Oct. 34/1: Written by Sylvia Robinson, three motor-mouthed disc jockeys ‘do their vocal thing’ as they rhyme in double time on subjects that strike a responsive chord with their teenage fans.
2000		N. Griffiths Grits 439: Ta quote tha mota-mouthed cunt, legs which are ‘hysterically disproportionate’.