A small, painful infection on the eyelid, or an enclosure for pigs.
For the eye infection: from Old English 'stīgend' (rising), referring to the swelling. For the pig enclosure: from Old English 'stig' (pen, hall). Both words converged in Middle English as 'stye', though they have completely different etymological origins.
It's a remarkable coincidence that two completely unrelated Old English words - one about swelling and one about enclosures - both evolved into the modern spelling 'sty', creating an accidental homonym that occasionally confuses even native speakers.
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