pastern (n.)
late 13c., "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 following vowel occurred 1500s. Sense extended (1520s) to part of the leg to which the tether was attached.