An archaic or poetic form of 'you' used as the object of a verb or preposition. It was the informal singular form in Middle and Early Modern English.
From Old English 'þē', the accusative and dative form of 'þū' (thou). It was the intimate, informal form of address, contrasting with the formal 'you', but gradually disappeared from standard English except in religious, poetic, and dialectal contexts.
Ironically, 'thee' and 'thou' were originally the informal, intimate forms of address - like modern 'tu' in French - but they now sound extremely formal to us because they survived mainly in religious and legal language!
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