ˈblack-mouth A black-mouthed person or animal; fig. a foul-mouthed person, a slanderer. 1642Lond. Apprent. Decl. in Harl. Misc. (Malh.) V. 307 As some blackmouths have uncharitably belched out against us.1656Trapp Comm. John ix. 28 Every black-mouth cast dirt upon Christ's disciples.