Poor mouth refers to someone who continually complains about a lack of money, often as an excuse for not contributing to charities, paying bills, etc.
She is a real poor mouth. She has a brand new car in her driveway, but always delays paying her rent. 她是个不折不扣的哭穷鬼。她家门口停着崭新的车，但总是拖着不付房租。
When used as a verb, the term poor-mouth means to lament or argue that one is too poor; plead poverty, or to declare (one's ability, power, position, etc.) to be inadequate or disappointing, sometimes as an intentional understatement.
We know you're just poor-mouthing your skill at playing bridge—you're a good player.(dictionary.com)