Webster's Revised Unabridged Dictionary · 1913

Ywis

Ywis , adverb

[Old English ywis, iwis, Anglo-Saxon gewis certain; akin to Dutch gewis, German gewiss, and English wit to know. See Wit to know, and Y-.]

Certainly; most likely; truly; probably. [Obsolete 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
Our ship, I wis, Shall be of another form than this. — Longfellow

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 compare Wit, to know.