"not one or the other," Middle English neither, naither, nether, from Old English nawþer, contraction of nahwæþer, literally "not of two," from na "no" (from PIE root *ne- "not") + hwæþer "which of two" (see whether). Spelling altered c. 1200 by association with either. Paired with nor from c. 1300; earlier with ne. Meaning "not in any case, in no case, not at all" is from 1550s. Also used in late Old English as a pronoun. As an adjective, "not either," mid-14c.