pastern (n.)

late 13c., pastron, "shackle fixed on the foot of a horse or other beast," from Old French pasturon (Modern French paturon), diminutive of pasture "shackle for a horse in pasture," from Vulgar Latin *pastoria, noun use of fem. of Latin pastorius "of herdsmen," from pastor "shepherd" (see pastor). Metathesis of -r- and the following vowel occurred 1500s. The original sense is obsolete; the meaning was extended by 1520s to the part of the horse's leg between the fetlock and the hoof, to which the tether was attached.