by 1717, in algebra textbooks, in phrase to the nth, a mathematical term indicating an indefinite number, in which n is an abbreviation for (whole) number (n.). Figurative (non-mathematical) use is by 1852.
In other European languages, identity between the indefinite article and the word for "one" remains explicit (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.
In texts of Shakespeare, etc., an as a word introducing a clause stating a condition or comparison conjunction is a reduced form of and in this now-archaic sense "if" (a usage first attested late 12c.), especially before it.
"quality or state of endless duration, continued uninterrupted existence for an indefinite period of time," late 14c., perpetuite, from Old French perpetuité "permanence, duration" (13c., Modern French perpétuité) and directly from Latin perpetuitatem (nominative perpetuitas) "uninterrupted duration, continuity, continuous succession," from perpetuus (see perpetual).
"a quibble, a nicety or subtlety," 1580s, obsolete, probably a corruption or contraction of Latin quidlibet "what you please," from quid "anything," neuter of indefinite pronoun quis "somebody, someone or other" (from PIE root *kwo-, stem of relative and interrogative pronouns) + libet "it pleases" (from PIE root *leubh- "to care, desire, love").