Meaning

adjective (of horses) afflicted with a swelling of the hock-joint

originFrom Late Middle English spaueyned, from spavein, spaveine (“swelling on horse’s leg causing lameness; disease causing lameness in horses”) + -ed (suffix forming adjectives; and the past tense and past participle forms of weak verbs). By surface analysis, spavin + -ed (suffix forming possessional adjectives from nouns, denoting having the objects represented by the nouns; and forming past tenses…

Etymology from Wiktionary, CC BY-SA 3.0