Green’s Dictionary of Slang

slanderous, malicious.

c.1698		B.E. Dict. Canting Crew n.p.: black-mouth foul, malicious, Railing, or Reflecting.
1772		Bridges Burlesque Homer (3rd edn) 23: At such a rate he rav’d and tore, / I’m sure you would have ta’en him for / One of St. Giles’s black-mouth’d bitches.