Definition of forsooth in US English:

forsooth

adverb

archaic, humorous
  • sentence adverb Indeed (often used ironically or to express surprise or indignation)

    ‘forsooth, there is no one I trust more’
    • ‘‘Ah, forsooth, and is it your worship indeed?‘cried the good dame.’
    definitely, certainly, unquestionably, undoubtedly, positively, without doubt, without a doubt, beyond any doubt, beyond doubt, beyond question, unmistakably, indubitably, undeniably, beyond the shadow of a doubt, surely, assuredly
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Origin

Old English forsōth (see for, sooth).

Pronunciation

forsooth

/fərˈsuθ//fərˈso͞oTH/