An archaic or formal way of saying 'yourself,' used mainly in old texts, religious writing, or when imitating old English.
From Old English 'þiself' or 'þyself,' combining 'thy' (your) and 'self.' The 'th-' sound shows it predates modern English spelling conventions.
If you saw 'thyself' in a Shakespeare play, you'd know the character was speaking to a single person in a serious or formal way—it's the singular 'you' from when English distinguished between singular and plural. Modern English flattened this into just 'you,' so we lost this nuance.
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