What is "y-"

Y- \Y-\, or I- \I-\ . [OE. y-, i-, AS. ge-, akin to D. & G. ge-, OHG. gi-, ga-, Goth. ga-, and perhaps to Latin con-; originally meaning, together. Cf. Com-, Aware, Enough, Handiwork, Ywis.] A prefix of obscure meaning, originally used with verbs, adverbs, adjectives, nouns, and pronouns. In the Middle English period, it was little employed except with verbs, being chiefly used with past participles, though occasionally with the infinitive. Ycleped, or yclept, is perhaps the only word not entirely obsolete which shows this use.

That no wight mighte it see neither yheere.
--Chaucer.

Neither to ben yburied nor ybrent.
--Chaucer.

Note: Some examples of Chaucer's use of this prefix are; ibe, ibeen, icaught, ycome, ydo, idoon, ygo, iproved, ywrought. It inough, enough, it is combined with an adjective. Other examples are in the Vocabulary. [1913 Webster] Spenser and later writers frequently employed this prefix when affecting an archaic style, and sometimes used it incorrectly.

perfective prefix, in yclept, etc.; a deliberate archaism, introduced by Spenser and his imitators, representing an authentic Middle English prefix, from Old English ge-, originally meaning "with, together" but later a completive or perfective element, from Proto-Germanic *ga- "together, with" (also a collective and intensive prefix), from PIE *kom "beside, near, by, with" (cognate with Sanskrit ja-, Latin com-, cum-; see com-). It is still living in German and Dutch ge-, and survives, disguised, in some English words (such as alike, aware, handiwork).\n

\nAmong hundreds of Middle English words it formed are yfallen, yhacked ("completely hacked," probably now again useful), yknow, ymarried, ywrought.