sty: English has two distinct words sty. The ‘pig enclosure’ [OE] is not recorded for certain as an independent word before the 13th century, but it occurs in compounds in Old English, and it is probably the same word as Old English stig ‘hall’ (source of English steward). It goes back ultimately to a prehistoric Germanic *stijam. The sty on one’s eye [17] denotes etymologically a ‘swelling’.
It comes from the now defunct styany ‘sty’. This was misinterpreted as ‘sty-oneye’, but in fact it was a compound formed from Middle English styan ‘swelling’ (a descendant of the present participle of Old English stīgan ‘rise’, which is related to modern English stair and stirrup) and eye. => steward; eye, stair, stirrup[sty etymology, sty origin, 英语词源]