The Collaborative International Dictionary of English v.0.48:

Either \Ei"ther\ ([=e]"[th][~e]r or [imac]"[th][~e]r; 277), a. &
   pron. [OE. either, aither, AS. [=ae]g[eth]er,
   [=ae]ghw[ae][eth]er (akin to OHG. [=e]ogiwedar, MHG.
   iegeweder); [=a] + ge + hw[ae][eth]er whether. See Each,
   and Whether, and cf. Or, conj.]
   1. One of two; the one or the other; -- properly used of two
      things, but sometimes of a larger number, for any one.
      [1913 Webster]

            Lepidus flatters both,
            Of both is flattered; but he neither loves,
            Nor either cares for him.             --Shak.
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            Scarce a palm of ground could be gotten by either of
            the three.                            --Bacon.
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            There have been three talkers in Great British,
            either of whom would illustrate what I say about
            dogmatists.                           --Holmes.
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   2. Each of two; the one and the other; both; -- formerly,
      also, each of any number.
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            His flowing hair
            In curls on either cheek played.      --Milton.
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            On either side . . . was there the tree of life.
                                                  --Rev. xxii.
                                                  2.
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            The extreme right and left of either army never
            engaged.                              --Jowett
                                                  (Thucyd).
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The Collaborative International Dictionary of English v.0.48:

Either \Ei"ther\, conj.
   Either precedes two, or more, co["o]rdinate words or phrases,
   and is introductory to an alternative. It is correlative to
   or.
   [1913 Webster]

         Either he is talking, or he is pursuing, or he is in a
         journey, or peradventure he sleepeth.    --1 Kings
                                                  xviii. 27.
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         Few writers hesitate to use either in what is called a
         triple alternative; such as, We must either stay where
         we are, proceed, or recede.              --Latham.
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   Note: Either was formerly sometimes used without any
         correlation, and where we should now use or.
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               Can the fig tree, my brethren, bear olive
               berries? either a vine, figs?      --James iii.
                                                  12.
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WordNet (r) 3.0 (2006):

either
    adv 1: after a negative statement used as an intensive meaning
           something like `likewise' or `also'; "he isn't stupid,
           but he isn't exactly a genius either"; "I don't know
           either"; "if you don't order dessert I won't either"