"joint in the hind leg of a horse or other quadruped," corresponding to the ankle-joint in man, mid-15c., earlier hockshin (late 14c.), from Old English hohsinu "sinew of the heel, Achilles' tendon," literally "heel sinew," from Old English hoh "heel" (in compounds, such as hohfot "heel"), from Proto-Germanic *hanhaz (source also of German Hachse "hock," Old English hæla "heel"), from PIE *kenk- (3) "heel, bend of the knee" (see heel (n.1)).
"Rhenish wine," 1620s, shortening of Hockamore, a corrupt Englishing of German Hochheimer, "(wine) of Hochheim" (literally "high-home"), town on the Main where wine was made; sense extended to German white wines in general.
"pawn, debt," 1859, American English, in hock, which meant both "in debt" and "in prison," from Dutch hok "jail, pen, doghouse, hutch, hovel," in slang use, "credit, debt."
When one gambler is caught by another, smarter than himself, and is beat, then he is in hock. Men are only caught, or put in hock, on the race-tracks, or on the steamboats down South. ... Among thieves a man is in hock when he is in prison. [G.W. Matsell, "Vocabulum," 1859]