anyoudaoicibaDictYouDict[an 词源字典]
an: [OE] The indefinite article in English is ultimately identical with the word one (as is the case, even more obviously, in other European languages – French un, German ein, and so on). The ancestor of both a(n) and one was ān, with a long vowel, but in the Old English period it was chiefly used for the numeral; where we would use a(n), the Anglo-Saxons tended not to use an article at all. Ān begins to emerge as the indefinite article in the middle of the 12th century, and it was not long before, in that relatively unemphatic linguistic environment, its vowel became weakened and shortened, giving an.

And at about the same time the distinction between an and a began to develop, although this was a slow process; until 1300 an was still often used before consonants, and right up to 1600 and beyond it was common before all words beginning with h, such as house.
=> one[an etymology, an origin, 英语词源]
anyoudaoicibaDictYouDict
indefinite article before words beginning with vowels, 12c., from Old English an (with a long vowel) "one; lone," also used as a prefix an- "single, lone;" 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 indefinite article and the word for "one" remains explicit (as in 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. Circa 15c., a and an commonly were written as one word with the following noun, which contributed to the confusion over how such words as newt and umpire ought to be divided (see N).

In Shakespeare, etc., an sometimes is a contraction of as if (a usage first attested c. 1300), especially before it.