eitheryoudaoicibaDictYouDict[either 词源字典]
either: [OE] Either is the modern descendant of an ancient Germanic phrase which meant literally ‘always each of two’. Its constituents were *aiwō, source of English aye ‘ever, always’, (which was also one of the building blocks of which each was made) and *gikhwatharaz, ancestor of English whether. In Old English this became lexicalized as the compound ǣgehwæther, subsequently contracted to ǣgther, from which developed modern English either. Despite its similarity, neither is more than just either with a negative prefix tacked on: its history is parallel but slightly different.
=> aye, whether[either etymology, either origin, 英语词源]
eitheryoudaoicibaDictYouDict
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 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.