Fetlock \Fet"lock\, n. [OE. fetlak, fitlock, cf. Icel. fet pace, step, fit webbed foot of water birds, akin to E. foot. [root]77. See Foot.]

The cushionlike projection, bearing a tuft of long hair, on the back side of the leg above the hoof of the horse and similar animals. Also, the joint of the limb at this point (between the great pastern bone and the metacarpus), or the tuft of hair.

Their wounded steeds Fret fetlock deep in gore. --Shak.

Source: Webster's Revised Unabridged Dictionary (1913)