"one-tenth of a cent," 1786, an original U.S. currency unit but now used only for tax calculation purposes, shortening of Latin millesimum "one-thousandth part," from mille "a thousand" (see million). Formed on the analogy of cent, which is short for Latin centesimus "one hundredth" (of a dollar). Compare mil.
word-forming element in nouns of act, process, function, condition, from Old French and French -age, from Late Latin -aticum "belonging to, related to," originally neuter adjectival suffix, from PIE *-at- (source of Latin -atus, past participle suffix of verbs of the first conjugation) + *-(i)ko-, secondary suffix forming adjectives (see -ic).
<a href="https://www.etymonline.com/word/millage">Etymology of millage by etymonline</a>
Harper, D. (n.d.). Etymology of millage. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/millage