fetlock

Fetlock is a Middle English compound: the first element, fet or fit, is related to Old Norse fet, step or foot, and Old English fōt, foot. The second element, -lock, refers to a tuft of hair — as in forelock, the tuft of hair at the front of the head. A fetlock is therefore, literally, the foot-tuft — the distinctive patch of hair at the back of a horse's pastern joint, just above the hoof. The word is a small description of anatomy compressed into a name.

The fetlock joint itself is anatomically the equivalent of the human ankle, but this is not immediately obvious because horses walk on what are effectively the tips of their toes. A horse's hoof corresponds to the human toenail; the pastern corresponds to the toe bones; the fetlock joint corresponds to the ball of the foot; and the knee is actually the wrist. Horse anatomy confounds human anatomical intuitions at every level.

The fetlock is a critical structure in equine lameness assessment. Because horses carry enormous weight on relatively slender legs, joint problems are among the most common causes of performance limitation and retirement. Veterinarians palpate the fetlock for heat and swelling, measure flexion angles, and use imaging to assess the sesamoid bones within the joint. A horse can be sound at the walk and lame at the trot; the fetlock may hold the difference.

In Shakespeare's Henry V, the king's horse Roan Barbary is described with equestrian precision; later, Shakespeare gives Falstaff the memorable image of a horse 'with a most dreadful fetlock.' The word was common in educated discourse because horses were central to daily life across all classes. Now it is specialist terminology, reserved for those who have reason to examine the lower limbs of horses closely.