Meaning of Y- in English
alt. of i-.
2. y- ·- 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.
Webster English vocab. Английский словарь Webster. 2012