early 15c., "a lobe of the liver or lungs," from Medieval Latin lobus "a lobe," from Late Latin lobus "hull, husk, pod," from Greek lobos "lobe, lap, slip; vegetable pod," used of lap- or slip-like parts of the body or plants, especially "earlobe," but also of lobes of the liver or lungs, a word of unknown origin. It is perhaps related to Greek leberis "husk of fruits," from PIE *logwos. Beekes writes that the proposed connection with the PIE source of English lap (n.1)) "is semantically attractive." Extended 1670s to divisions of the brain; 1889 to ice sheets. The common notion is "rounded protruding part."

c. 1300, majour, "greater, more important or effective, leading, principal," from Latin maior (earlier *magios), irregular comparative of magnus "large, great" (from PIE root *meg- "great"). From 1590s as "greater in quantity, number, or extent." Used in music (of modes, scales, or chords) since 1690s, on notion of an interval a half-tone "greater" than the minor; of modern modes, "characterized by the use of major tonality throughout," by 1811. Major league, in baseball, is attested by 1892.

military rank above captain and below lieutenant colonel, 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.)).

His chief duties consist in superintending the exercises of his regiment or battalion, and in putting in execution the commands of his superior officer. His ordinary position in the line is behind the left wing. [Century Dictionary, 1897]

The musical sense is attested by 1797.

of a college or university student, "focus (one's) studies," 1910, American English, from major (n.) in sense of "subject of specialization" (by 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] a sense derived from the military major.

"military officer next in rank below a lieutenant-general," 1640s; see major (n.) + general (n.).

also majordomo, "man employed to superintend a household, especially that of a sovereign or other dignitary," 1580s, via Italian maggiordomo or Spanish mayordomo, from Medieval Latin major domus "chief of the household," also "mayor of the palace" under the Merovingians, from Latin maior "greater" (see major (adj.)) + genitive of domus "house" (from PIE root *dem- "house, household").

"having lobes," 1760, from Modern Latin lobatus "lobed," from lobus "a lobe" (see lobe). Related: Lobation.

also ear-lobe, "the lobe of the ear," by 1786, from ear (n.1) + lobe. Earlier was ear lap (Old English had earlæppa "external ear").

"of or pertaining to a lobe or lobes," 1839, from modern Latin lobaris, from Latin lobus (see lobe (n.)).

extinct marine arthropod, 1820, from Modern Latin Trilobites (Walch, 1771), from Greek tri- "three" (see three) + lobos "lobe" (see lobe); so called because its body is divided into three lobes.