/ ðaɪ; NAmE / (also thine / ðaɪn; NAmE / before a vowel) determiner
( old use ) a word meaning 'your', used when talking to only one person :
Honour thy father and thy mother.
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WORD ORIGIN
Middle English thi (originally before words beginning with any consonant except h ), reduced from thin , from Old English thīn , of Germanic origin.