'thine' originates from Old English, specifically the word 'þīn', where 'þīn' meant 'your' or 'yours'.

'þīn' changed from Old English to Middle English 'thine' and eventually became the modern English word 'thine'.

Initially, it meant 'your' or 'yours', and this meaning has largely remained the same in modern usage, though it is now considered archaic.