The Collaborative International Dictionary of English v.0.48:

Wis \Wis\, adv. [Aphetic form of iwis, ywis; or fr. Icel. viss
   certain. See Ywis.]
   Certainly; really; indeed. [Obs.] "As wis God helpe me."
   --Chaucer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Wis \Wis\, v. t. [Due to mistaking OE. iwis certain, AS. gewiss,
   for I wis. See Ywis.]
   To think; to suppose; to imagine; -- used chiefly in the
   first person sing. present tense, I wis. See the Note under
   Ywis. [Obs. or Poetic] "Howe'er you wis." --R. Browning.
   [1913 Webster]

         Nor do I know how long it is
         (For I have lain entranced, I wis).      --Coleridge.
   [1913 Webster]