Real Dictionary

poor mouth in American English
noun: a person who continually complains about a lack of money

poor mouth in British English
noun: unjustified complaining, esp to excite sympathy
verb: to speak of disparagingly; decry

poor-mouth in American English
to complain about one's lack of money; plead poverty
intransitive verb: to lament or argue that one is too poor; plead poverty
transitive verb: to declare (one's ability, power, position, etc.) to be inadequate or disappointing, sometimes as an intentional understatement; downplay

poor-mouth in British English
to speak of disparagingly; decry