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O.E. ægðer, contraction of æghwæðer "each of two, both," from a "always" + ge- collective prefix + hwæðer "which of two, whether." Modern sense of "one or the other of two" is early 14c. Use of either-or to suggest an unavoidable choice between alternatives (1931) in some cases reflects Dan. enten-eller, title of an 1843 book by Kierkegaard.