Etymology
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either

Old English ægðer, contraction of æghwæðer (pron., adv., conj.) "each of two, both," from a "always" (see aye (adv.)) + ge- collective prefix + hwæðer "which of two, whether" (see whether). Cognate with Old Frisian eider, Dutch ieder, Old High German eogiwedar, German jeder "either, each, every").

Modern sense of "one or the other of two" is late 13c. Adverbially, for emphasis, "in any case, at all," especially when expressing negation, by 1828. Use of either-or to suggest an unavoidable choice between alternatives (1931) in some cases reflects Danish enten-eller, title of an 1843 book by Kierkegaard.

updated on July 25, 2018

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Definitions of either from WordNet

either (adv.)
after a negative statement used as an intensive meaning something like `likewise' or `also';
if you don't order dessert I won't either
I don't know either
he isn't stupid, but he isn't exactly a genius either
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.