Green’s Dictionary of Slang

horse’s ass n.

also horse-ass, horse’s arse, ...behind, ...can, ...cock, ...foot, ...rear
[SE horse + ass n. (2)/arse n. (1)/behind n. (1)/can n.1 (1b)/cock n.3 (1)]
[mid-19C+]

1. a fool, an idiot.

2. a general term of abuse.