minor (n.) Look up minor at Dictionary.com
early 14c., "a Franciscan," from Latin Fratres Minores "lesser brethren," name chosen by St. Francis, who founded the order, for the sake of humility; see minor (adj.). From c.1400 as "minor premise of a syllogism." From 1610s as "person under legal age" (Latin used minores (plural) for "the young"). Musical sense is from 1797. Meaning "secondary subject of study, subject of study with fewer credits than a major" is from 1890; as a verb in this sense from 1934.
minor (adj.) Look up minor at Dictionary.com
early 13c., menour "Franciscan" (see minor (n.)), from Latin minor "less, lesser, smaller, junior," figuratively "inferior, less important," formed as a masculine/feminine form of minus on the mistaken assumption that minus was a neuter comparative, from PIE root *mei- (2) "small" (see minus).

Some English usages are via Old French menor "less, smaller, lower; underage, younger," from Latin minor. Meaning "underage" is from 1570s. Meaning "lesser" in English is from early 15c.; that of "less important" is from 1620s. The musical sense is from 1690s. In the baseball sense, minor league is from 1884; the figurative extension is first recorded 1926.