Another

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Middle English: as an other until the 16th century.

wiktionary

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From Middle English another, equivalent to an +‎ other.

etymonline

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another (pron., adv.)

"not this, not the same; someone or something else," early 13c., merger of an + other. Old English used simply oþer. Originally "a second of two." Compound reciprocal pronoun one another is recorded from 1520s.