"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.
also main-line, "inject (drugs) intravenously," 1934, from main line in American English slang sense "principal vein into which drugs can be injected" (1933).
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.
late 14c., "to tie with a cord," from line (n.). Meaning "to mark or mark off with lines" is from mid-15c. Sense of "arrange a line" is from 1640s, originally military; that of "to join a line" is by 1773. To line up is by 1864 as "form a good line, be in alignment;" 1889 as "form a line," in U.S. football; transitive sense "make into a line" is by 1902. Also see line-up. For line bees see bee-line. Related: Lined; lining.
a Middle English merger of Old English line "cable, rope; series, row, row of letters; rule, direction," and Old French ligne "guideline, cord, string; lineage, descent" (12c.), both from Latin linea "linen thread, string, plumb-line," also "a mark, bound, limit, goal; line of descent," short for linea restis "linen cord," and similar phrases, from fem. of lineus (adj.) "of linen," from linum "linen" (see linen).
The earliest sense in Middle English was "cord used by builders for taking measurements;" extended late 14c. to "a thread-like mark" (from sense "cord used by builders for making things level," mid-14c.), also "track, course, direction." Meaning "limit, boundary" (of a county, etc.) is from 1590s. The mathematical sense of "length without breadth" is from 1550s. From 1530s as "a crease of the face or palm of the hand." From 1580s as "the equator."
Sense of "things or people arranged in a straight line" is from 1550s. Now considered American English, where British English uses queue (n.), but the sense appears earliest in English writers. Sense of "chronologically continuous series of persons" (a line of kings, etc.) is from late 14c.
Meaning "one's occupation, branch of business" is from 1630s, according to OED probably from misunderstood KJV translation of II Corinthians x.16, "And not to boast in another mans line of things made ready to our hand," where line translates Greek kanon which probably meant "boundary, limit;" the phrase "in another man's line" being parenthetical.
Commercial meaning "class of goods in stock" is from 1930, so called from being goods received by the merchant on a line in the specific sense "order given to an agent" for particular goods (1834). Insurance underwriting sense is from 1899. Line of credit is from 1958.
Meaning "series of public conveyances" (coaches, later ships) is from 1786; meaning "continuous part of a railroad" is from 1825. Meaning "telegraph wire between stations" is from 1847 (later "telephone wire"). Meaning "cord bearing hooks used in fishing" is from c. 1300. Meaning "policy or set of policies of a political faction" is 1892, American English, from notion of a procession of followers; this is the sense in the political party line, and, deteriorated, it is the slang line that means "glib and plausible talk meant to deceive."
In British army, the Line (1802) is the regular, numbered troops, as distinguished from guards, auxiliaries, militia, etc. In the Navy (1704) it refers to the battle line (the sense in ship of the line, which is attested from 1706).
Dutch lijn, Old High German lina, German Leine, Old Norse lina "a cord, rope," are likewise from Latin. Spanish and Italian have the word in the learned form linea. In continental measurements, a subdivision of an inch (one-tenth or one-twelfth in England), attested in English from 1660s but never common. Also see lines.
To get a line on "acquire information about" is from 1903. To lay it on the line is from 1929 as "to pay money;" by 1954 as "speak plainly." End of the line "as far as one can go" is from 1948. One's line of work, meaning "pursuit, interest" is from 1957, earlier line of country (1861). Line-drawing is from 1891. A line-storm (1850) is a type supposed to happen in the 10 days or two weeks around the times the sun crosses the equator.
"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"]