y- Look up y- at Dictionary.com
perfective prefix, in y-clept, etc.; a deliberate archaism, introduced by Spenser and his imitators, representing an authentic M.E. prefix, from O.E. ge-, originally meaning "with, together" but later a completive or perfective element, from P.Gmc. *ga-. It is still living in Ger. and Du. ge-, and survives, disguised, in some Eng. words (e.g. alike, aware).