Old English mægen (Mercian megen) "power, bodily strength; force, violent effort; strength of mind or will; efficacy; supernatural power," from Proto-Germanic *maginam "power" (source also of Old High German megin "strength, power, ability"), suffixed form of PIE root *magh- "to be able, have power."
Original sense of "power" is preserved in phrase might and main. Also used in Middle English for "royal power or authority" (c. 1400), "military strength" (c. 1300), "application of force" (c. 1300). Meaning "chief or main part" (c. 1600) now is archaic or obsolete. Meaning "principal duct, pipe, or channel in a utility system" is first recorded 1727 in main drain.
Used since 1540s for "continuous stretch of land or water;" in nautical jargon used loosely for "the ocean," but in Spanish Main the word is short for mainland and refers to the coast between Panama and Orinoco (as contrasted to the islands of the West Indies).
early 13c., "notably large, bulky, or strong" (a sense now obsolete), from Old English mægen- "power, strength, force," used in compounds (such as mægensibb "great love," mægenbyrðen "heavy burden;" see main (n.)), probably also in part from or influenced by cognate Old Norse megenn (adj.) "strong, powerful, mighty."
Sense of "chief, principal, prime" is from c. 1400. That of "principal or chief in size or extent" is from 1590s. Main chance "opportunity of enriching oneself" is by 1570s, from the game of hazard. Main course in the meal sense attested from 1829. Main man "favorite male friend; hero" is by 1967, African-American vernacular.
"principal street of a (U.S.) town," 1810, from main (adj.) + street. Used allusively to indicate "mediocrity, small-town materialism" from late 19c., a sense reinforced by the publication of Sinclair Lewis's novel "Main Street" (1920).
But a village in a country which is taking pains to become altogether standardized and pure, which aspires to succeed Victorian England as the chief mediocrity of the world, is no longer merely provincial, no longer downy and restful in its leaf-shadowed ignorance. It is a force seeking to dominate the earth, to drain the hills and sea of color, to set Dante at boosting Gopher Prairie, and to dress the high gods in Klassy Kollege Klothes. Sure of itself, it bullies other civilizations, as a traveling salesman in a brown derby conquers the wisdom of China and tacks advertisements of cigarettes over arches for centuries dedicate to the sayings of Confucius. ["Main Street"]
"principal line of a railway," 1841, from main (adj.) + line (n.). Meaning "affluent area of residence" is by 1917, originally (with capitals) that west of Philadelphia, from the "main line" of the Pennsylvania Railroad which added local stops to a string of backwater towns west of the city late 19c. that helped turn them into fashionable suburbs.
The Main Line, Philadelphia's most famous suburban district, was deliberately conceived in the 1870's and 1880's by the [Pennsylvania] Railroad, which built high-toned housing developments, ran hotels, more or less forced its executives to plunk their estates out there, and created a whole series of somewhat spurious Welsh towns along the railroad tracks. ... Now everybody assumes these all date from 1682, like the Robertses; but as Chestnut Hill people like to say, "nobody but Welsh peasants lived on the Main Line till the Railroad built it up." [Nathaniel Burt, "The Perennial Philadelphians," 1963]
The original station stops were, in order out from the city, Overbrook, Merion, Narberth, Wynnewood, Ardmore, Haverford, Bryn Mawr, Paoli. The train line for commuters along it is the Paoli Local.