Green’s Dictionary of Slang

(US) a stupid, incompetent person.

1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 397: ‘I’m one of the halfbacks,’ said Phil. ‘Sure, he’s the All-American-Half-Ass,’ said Studs.
1955		E. Hoagland Cat Man 225: Brownie shriveled the townies. He let them have the benefit of his advice. ‘You half-asses are getting out of here if I have to goose the jizz out of every one of you’.
1989		H. Rawson Dict. of Invective (1991) 22: Commonly encountered subspecies, in addition to the previously mentioned smart ass, include complete ass, dumb ass, half ass, horse’s ass, stupid ass, and wise ass.