The Collaborative International Dictionary of English v.0.48:

Ywis \Y*wis"\, adv. [OE. ywis, iwis, AS. gewis certain; akin to
   D. gewis, G. gewiss, and E. wit to know. See Wit to know,
   and Y-.]
   Certainly; most likely; truly; probably. [Obs. or Archaic]
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         "Ywis," quod he, "it is full dear, I say." --Chaucer.
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         She answered me, "I-wisse, all their sport in the park
         is but a shadow to that pleasure that I find in Plato."
                                                  --Ascham.
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         A right good knight, and true of word ywis. --Spenser.
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   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.
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               Our ship, I wis,
               Shall be of another form than this. --Longfellow.
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