According to the ONLINE ETYMOLOGY DICTIONARY**, the etymological “definition” of the indefinite article “a” is:
a form of an used before consonants, mid-12c., a weakened form of Old English an “one” (see an). The disappearance of the -n- before consonants was mostly complete by mid-14c. After c. 1600 the -n- also began to vanish before words beginning with a sounded -h-; it still is retained by many writers before unaccented syllables in h- or (e)u- but is now no longer normally spoken as such. The -n- also lingered (especially in southern England dialect) before -w- and -y- through 15c.
It also is used before nouns of singular number and a few plural nouns when few or great many is interposed.
For reasons unclear, I wondered all of a sudden how that much overworked and under-appreciated word “a” came about…