The joint on a horse's or other four-legged animal's leg just above the hoof where the fetlock bone connects; also, the tuft of hair that grows above this joint.
From Middle English 'fetelock,' possibly derived from Old English 'fet' (foot) and 'loc' (lock or tuft of hair). The term has referred to this specific horse anatomy since medieval times.
Horse anatomy got named so literally—'fetlock' literally means the lock (tuft) of hair at the foot—yet this medieval term is still the official term for that exact leg joint used by veterinarians and riders today.
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