a person who is a constant or irrepressible talker.
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How to use motor-mouth in a sentence
In his previous roles, the 26-year-old actor was like Vince Vaughn-lite—a charismatic, eminently likeable motor mouth.‘Whiplash’ Is Sundance’s Hottest Film, A Music-Themed Drama Starring Miles Teller and J.K. Simmons | Marlow Stern | January 24, 2014 | THE DAILY BEAST
British Dictionary definitions for motormouth
slang a garrulous person
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012