An archaic second-person singular form of 'be,' used in older English (as in 'thou bist').
From Old English bist, the second-person singular past tense of 'be.' This form appears in Early Modern English texts like the King James Bible and Shakespeare, derived from proto-Germanic roots.
If you were a peasant in Shakespeare's time talking to someone important, you'd say 'thou bist' (you are), but if they were doing the same to you, you'd use different grammar—English used to track respect levels through word endings!
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