Source : Webster's Revised Unabridged Dictionary (1913)

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]

         ``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.