Y·wis adv.  Certainly; most likely; truly; probably.  [Obs. or Archaic]
     =\“Ywis,” quod he, “it is full dear, I say.”\=   --Chaucer.
     She answered me, =\“I-wisse, all their sport in the park is but a shadow to that pleasure that I find in Plato.”\=   --Ascham.
     A right good knight, and true of word ywis.   --Spenser.
  Note: ☞ The common form iwis was often written with the prefix apart from the rest of the word and capitalized, as, I wis, I wisse, etc. The prefix was mistaken for the pronoun, I and wis, wisse, for a form of the verb wit to know.  See Wis, and cf. Wit, to know.
  Our ship, I wis,
  Shall be of another form than this.   --Longfellow.
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