An archaic interjection meaning 'in truth' or 'indeed'; used to express emphasis, surprise, or scorn.
From Old English 'for soth,' where 'soth' means truth. Originally meant literally 'in truth' but became primarily archaic and theatrical by the Early Modern period.
'Forsooth!' is the classic word actors use in Shakespearean performances, but real medieval people said it seriously—it was like modern 'honestly!' or 'I swear!'
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