"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.

"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"]

figurative sense is attested from 1832, from profit-and-loss accounting, where the final figure calculated is the bottom line on the page. Also (especially as an adjective) bottom-line, bottomline.

1590s, in reference to the horizontal and vertical lines of soldiers marching in formation, from rank (n.) in the military sense of "number of soldiers drawn up in a line abreast" (1570s) + file (n.1). Thence generalized to "common soldiers" (1796) and "common people, general body" of any group (1860).

on the Great Seal of the United States of America, apparently an allusion to line 5 of Virgil's "Eclogue IV," in an 18c. edition: Magnus ab integro seclorum nascitur ordo "The great series of ages begins anew." The seal's designer, Charles Thomson, wrote that the words "signify the beginnings of the New American Era." (see Annuit Coeptis).

1906, named for U.S. president Theodore "Teddy" Roosevelt (1858-1919), a noted big-game hunter, whose conservationist fervor inspired a comic illustrated poem in the New York Times of Jan. 7, 1906, about two bears named Teddy, whose names were transferred to two bears presented to the Bronx Zoo that year. The name was picked up by toy dealers in 1907 for a line of "Roosevelt bears" imported from Germany. Meaning "big, lovable person" first attested 1957, from the song popularized by Elvis Presley.