Thy

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Middle English thi (originally before words beginning with any consonant except h ), reduced from thin, from Old English thīn (see thine).

wiktionary

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From Middle English þi, apocopated variant of þin, from Old English þīn, from Proto-West Germanic *þīn, from Proto-Germanic *þīnaz, from Proto-Indo-European *téynos(“thy; thine”), from Proto-Indo-European *túh₂(“thou”). See thou.

thy

etymonline

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thy (pron.)

possessive pronoun of 2nd person singular, late 12c., reduced form of þin (see thine), until 15c. used only before consonants except -h-. Compare my/ mine, a/ an.