"like one another, very similar," c. 1300, aliche, ylike, ilike, from Old English anlig, onlic "similar, resembling;" from Old English an, on (see a- (1) + like (adj.), which is related to Old English lic "body, corpse."
The notion is "having a corresponding form (body)." The more usual Germanic compound is represented by Old English gelic, from Proto-Germanic *galikam "associated form" (source also of Old Frisian gelik, Dutch gelijk, German gleich, Gothic galeiks, Old Norse glikr). As an adverb, late Old English onlice, gelice.