another pron., adv.

indefinite article before words beginning with vowels, 12c., from Old English an (with a long vowel) "one; lone," also used as a prefix meaning "single, lone" (as in anboren "only-begotten," anhorn "unicorn," anspræce "speaking as one"). See one for the divergence of that word from this. Also see a, of which this is the older, fuller form.

In other European languages, identity between the indefinite article and the word for "one" remains explicit (French un, German ein, etc.). Old English got by without indefinite articles: He was a good man in Old English was he wæs god man.

In texts of Shakespeare, etc., an as a word introducing a clause stating a condition or comparison conjunction is a reduced form of and in this now-archaic sense "if" (a usage first attested late 12c.), especially before it.

Old English oþer "second, the second of two; additional, further" (adj.), also as a pronoun, "one of the two; a different person or thing from the one in view," from Proto-Germanic *anthera- (source also of Old Saxon athar, Old Frisian other, Old Norse annarr, Middle Dutch and Dutch ander, Old High German andar, German ander, Gothic anþar "second, other").

These are from PIE *an-tero-(source of Lithuanian antras, Old Prussian anters "other, second), which is perhaps a variant of *al-tero- "the other of two" (source of Latin alter), from root *al- "beyond" + adjectival comparative suffix *-tero-.  Or the first element might be the pronoun *eno-, *ono- [Boutkan]. The Old English, Old Saxon, and Old Frisian forms show "a normal loss of n before fricatives" [Barnhart].

The sense of "second" was detached from this word in English (which now uses second (adj.), which is from Latin) and German (zweiter, from zwei "two") to avoid ambiguity. In Scandinavian, however, the second floor is still the "other" floor (Swedish andra, Danish anden). Also compare Old English oþergeara "next year."

As an adverb, "secondly" (late Old English); "otherwise" (c. 1200); "in addition" (mid-14c.).

The other woman "a woman with whom a man begins a love affair while he is already committed" is from 1855. The other day originally (late Old English) was "the next day;" later (c. 1300) "yesterday;" and now, loosely, "a day or two ago" (early 15c.). OED notes that the other place was euphemistic for Hell or "Oxford as regarded in Cambridge (and vice versa)." Phrase other half in reference to either the poor or the rich, is recorded from c. 1600.

La moitié du monde ne sçayt comment l'aultre vit. [Rabelais, "Pantagruel," 1532]

Halfe the world knowes not how the other halfe li[v]es. [George Herbert, "Outlandish Proverbs," 1640]

word formed from misdivision of another as a nother (see N for other examples), c. 1300. From 14c.-16c. no nother is sometimes encountered as a misdivision of none other or perhaps as an emphatic negative; Old English had noðer as a contraction of ne oðer "no other." Hence Middle English nother-gates (adv.) "not otherwise" (c. 1300).