Green’s Dictionary of Slang

(US) a whinger, a complainer.

[1830		‘Catalogue of Odd Fish’ in Fleet-Street Collection 8: Shotten herring’s a man when he lacks work. [...] A carp of your critic race smacks].
1908		‘O. Henry’ in Works 308: The town [is] stocked full of carp.
1912		Pedagogical Seminary Mar. 96: ‘Bonehead’...‘you carp,’ ‘you’re off your perch.’ .
1932		P.G. Cressey Taxi-Dance Hall 101: They’re such easy ‘carp’ I figure that if they hang around long enough somebody’s bound to knock them off.