Green’s Dictionary of Slang

(US) weary, depressed, ‘down in the mouth’, whether or not genuinely so.

1952		H. McCoy Corruption City 29: Mike put on a very poor-mouth face.
1974		V.E. Smith Jones Men 135: We been hearin’ this kinda poormouth shit for a week.
1986		S. King It (1987) 467: Hitting that poormouth sonofabitch didn’t give me a bit of pleasure.
2012		J. Hodgkins ‘Peek-a-Boo’ in ThugLit Dec. [ebook] ‘Any poormouth excuse you’d like to offer up?’.