Green’s Dictionary of Slang

hock n.1

[SE hock, a joint in the back leg of a quadruped, between the knee and the fetlock, which points backwards; curby f. curb, a disease of horses manifested in a swelling on the hock]

[late 18C+] the foot, or the foot and ankle; usu. in pl.; thus curby hocks n., round or clumsy feet.

In phrases

rattle one’s hocks (v.)

[1900s] (US) to go fast.