An archaic or biblical second-person singular past tense of 'to be,' used in older English as in 'thou wast' (you were).
From Old English 'wære,' the past tense of 'be.' The '-st' ending indicates second person (thou/you), showing how English once had detailed verb conjugations that we've now simplified.
Modern English has almost completely lost second-person verb forms—we just say 'you were' regardless of number—but 300 years ago you'd change the verb for 'I was,' 'thou wast,' 'he/she was,' showing how language simplifies over time by dropping distinctions that seem unnecessary.
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