'a' originates from Old English 'ān' (also the source of 'an'), ultimately from Proto-Germanic '*ainaz' meaning 'one'.

'ān' in Old English developed into Middle English forms 'an' and 'a' (with vowel reduction). The numeral 'one' and the indefinite article diverged in form and use: the article commonly reduced to a schwa sound.

Initially it meant 'one' (the numeral). Over time it evolved into an indefinite article meaning 'one (unspecified)' and often reduced in pronunciation to a weak form.