Green’s Dictionary of Slang

(US) cheeky, disrespectful.

1984		W.D. Myers Motown and Didi 19: He had already got into his smart-mouth ways.
1992		J. Mowry Way Past Cool 19: More like smart-mouth little niggerboys.
1999		Guardian Guide 12–18 June 19: An entertaining girl-coming-of-age movie, starring smart-mouthed Natasha Lyonne.
2000		F.X. Toole Rope Burns 42: Smart-mouth brother start crying in the corner when he see what Reggie be doing to his baby brother face and ribs.
2003		A.N. LeBlanc Random Family 242: Smart-mouthed toddlers using smart-ass words.
2009		L. McIlvanney All the Colours 260: He’d [...] got smart-mouthed in two or three of the wrong pubs and said some careless things.
2011		J.J. DeCeglie Drawing Dead [ebook] He’s a bona fide smartmouth shithead.