Green’s Dictionary of Slang

1. (also smarty-mouth) an insolent person.

1968		Sun Mag. (Baltimore) 13 Oct. 19/1: I was a smart mouth, a troublemaker in school [OED].
1980		E. Folb Runnin’ Down Some Lines.
1993		J. Mowry Six Out Seven (1994) 209: I got me a nice new batch of yellow soap ain’t never been tasted by a smarty-mouth yet!

2. cheek, insolence.

1983		S. King Christine 55: Don’t give me any of your smartmouth, kid.
1993		J. Mowry Six Out Seven (1994) 63: He didn’t take to smart-mouth from black nor white.
1999		J. Ridley Everybody Smokes in Hell 7: The automatic punch to the head that was likely to follow that brand of smart mouth.
2007		N. Bush Ultraviolet 307: ‘You’ve got a smart mouth.’ ‘Yeah . . . well ...’ I murmured, my smart mouth getting dumber.