The word 'one' worn down by centuries of unstressed use — from Old English 'an,' from PIE *hoynos (one).
The indefinite article, used before a singular noun to refer to any member of a class or to something not previously mentioned.
From Middle English 'a,' reduced form of 'an,' from Old English 'ān' (one), from Proto-Germanic *ainaz (one), from PIE *h₂óynos (one, single). The indefinite article did not exist in Old English — 'ān' was simply the numeral 'one.' During the Middle English period, the unstressed form of 'one' weakened to 'an' and then 'a' before consonants