c.1300, from Latin maior (earlier *magjos), irregular comparative of magnus "large, great" (see magnate). Used in music (of modes, scales, or chords) since 1690s, on notion of an interval a half-tone greater than the minor.
military rank, 1640s, from French major, short for sergent-major, originally a higher rank than at present, from Medieval Latin major "chief officer, magnate, superior person," from Latin maior "an elder, adult," noun use of the adjective (see major (adj.)). The musical sense attested by 1797.
"focus (one's) studies," 1910, American English, from major (n.) in sense of "subject of specialization" (1890). Related: Majored; majoring. Earlier as a verb, in Scottish, "to prance about, or walk backwards and forwards with a military air and step" [Jamieson, 1825].