Green’s Dictionary of Slang

poop n.³

a fool.

1893		F. Remington letter 13 Nov. in Splete Sel. Letters (1988) 167: He’s good for soldiers but he would make life hell for poops & doods.
1915		V. Woolf Voyage Out 51: They talk about art, and think us such poops for dressing in the evening [OED].
1926		‘Sapper’ Final Count 839: They think I’m a positive poop — a congenital what-not.
1934		Wodehouse Right Ho, Jeeves 129: We must always keep in mind the fact that we are dealing with a poop.
1942		P. Wylie Generation of Vipers 96: Poops and prickamice of every description have got themselves public offices, fortunes, and even what passes for literary reputations.
1954		K. Vonnegut ‘A Present for Big Saint Nick’ in Bagombo Snuff Box (1999) 168: They danced out of the house without their coats, saying things like, ‘Jingle bells, you old poop’.
1960		J.D. Macdonald Slam the Big Door (1961) 85: Those old poops that live there must’ve got a real earful that night.
1978		J. Dingwall Sun. Too Far Away 114: You stupid poop, how much did you lose?
1989		Kirk & Madsen After The Ball 303: So one virtually must act out one’s most fleeting impulses in order to prove that one isn’t a hung-up old poop.