Green’s Dictionary of Slang

to belittle oneself or others; to pose as impoverished.

1910		P.W. Joyce Eng. As We Speak It In Ireland.
1968		Atlantic Monthly Oct. 70: ‘I sell a few hides to pay the taxes,’ he poor-mouthed.
1972		G.V. Higgins Friends of Eddie Coyle 85: I’m always handing him twenty and he’s always poor-mouthing me.
1985		J. Wambaugh Secrets of Harry Bright (1986) 179: I hate poor-mouthing! Gimme your phone number so I can call in a pledge!
1990		J. Wambaugh Golden Orange (1991) 75: Well, you can stop poor-mouthing even if it’s true.
2004		R. Phalon Forbes Greatest Investing Stories 111: All told, Deangelis poormouthed his liquid assets to less than $5000, including ‘maybe six or seven suits.’.