Source : Webster's Revised Unabridged Dictionary (1913)

Either \Ei"ther\ (?; 277), a. & pron. [OE. either, aither, AS.
   ?g?er, ?ghw[ae]?er (akin to OHG. ?ogiwedar, MHG. iegeweder);
   [=a] + ge + hw[ae]?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.

            Lepidus flatters both, Of both is flattered; but he
            neither loves, Nor either cares for him. --Shak.

            Scarce a palm of ground could be gotten by either of
            the three.                            --Bacon.

            There have been three talkers in Great British,
            either of whom would illustrate what I say about
            dogmatists.                           --Holmes.

   2. Each of two; the one and the other; both; -- formerly,
      also, each of any number.

            His flowing hair In curls on either cheek played.
                                                  --Milton.

            On either side . . . was there the tree of life.
                                                  --Rev. xxii.
                                                  2.

            The extreme right and left of either army never
            engaged.                              --Jowett
                                                  (Thucyd).

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.

         Either he is talking, or he is pursuing, or he is in a
         journey, or peradventure he sleepeth.    --1 Kings
                                                  xviii. 27.

         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.

   Note: Either was formerly sometimes used without any
         correlation, and where we should now use or.

               Can the fig tree, my brethren, bear olive
               berries? either a vine, figs??     --James iii.
                                                  12.

Source : WordNet®

either
     adv : 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"