either

Etymology

From Middle English either, from Old English ǣġhwæþer, from Proto-Germanic, ultimately corresponding to ay (“always, ever”) + whether. Akin to Old Saxon eogihwethar, iahwethar (Low German jeed); Old Dutch *iogewether, *iowether, *iother (Dutch ieder); Old High German eogihwedar, iegihweder, ieweder (German jeder).

det

1. Any one (of two).
   You can have it in either colour.
2. Each of two; both.
   There is a locomotive at either end of the train, one pulling and the other pushing.
   Her hands, long and beautiful, lay on either side of her face. 1936, Djuna Barnes, Nightwood, Faber & Faber, published 2007, page 31
3. (now rare) Any one (of more than two).
   I hope you will be ready to own publicly, whenever you shall be called to it, that by your great and frequent urgency you prevailed on me to publish a very loose and uncorrect account of my travels, with directions to hire some young gentleman of either university to put them in order, and correct the style, as my cousin Dampier did, by my advice, in his book called “A Voyage round the world.” Jonathan Swift (1726) Gulliver's Travels, 1st edition

pron

1. One or the other of two people or things.
   He made me two offers, but I did not accept either.
   Hodgson may now have to bring in James Milner on the left and, on that basis, a certain amount of gloss was taken off a night on which Welbeck scored twice but barely celebrated either before leaving the pitch angrily complaining to the Slovakian referee. 7 September 2013, Daniel Taylor, “Danny Welbeck leads England's rout of Moldova but hit by Ukraine ban”, in The Guardian
2. (obsolete) Both, each of two or more.
   There have been three famous talkers in Great British, either of whom would illustrate what I say about dogmatists. 1872, Oliver Wendell Holmes, Sr., The Poet at the Breakfast-Table

adv

1. (conjunctive, after a negative) As well.
   I don't like him, and I don't like her either.
   I know a cheap Spanish restaurant. It's not far from here, either.
   But Richmond[…]appeared to lose himself in his own reflections. Some pickled crab, which he had not touched, had been removed with a damson pie; and his sister saw[…]that he had eaten no more than a spoonful of that either. 1959, Georgette Heyer, chapter 1, in The Unknown Ajax

conj

1. Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”.
   Either you eat your dinner or you go to your room.
   You can have either potatoes or rice with that, but not both.
   You'll be either early, late, or on time.
   You can't be a table and a chair. You're either a Jew or a gentile. December 5, 2006, Jackie Mason, quotee, “Mason drops lawsuit vs. Jews for Jesus”, in USA Today
   Thus, when he drew up instructions in lawyer language[…]his clerks[…]understood him very well. If he had written a love letter, or a farce, or a ballade, or a story, no one, either clerks, or friends, or compositors, would have understood anything but a word here and a word there. 1893, Walter Besant, “Prologue”, in The Ivory Gate