Etymology
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mainly (adv.)

c. 1300, "vigorously, strongly, forcibly" (a sense now obsolete), from main (adj.) + -ly (2). Meaning "especially, chiefly, principally" is from c. 1400; that of "for the most part" is from 1660s.

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mainmast (n.)

also main-mast, "the tallest mast in a sailing ship," late 15c., from main (adj.) + mast (n.1). In three-masted vessels, the middle mast; in four-masted ships the second from the bow.

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mainsail (n.)

also main-sail, in a square-rigged vessel, the sail bent to the main-yard, mid-15c., see main (adj.) + sail (n.).

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mainspring (n.)

1590s, "principal spring of a mechanism" (of a watch, clock, gun, etc.), from main (adj.) + spring (n.3). Figurative meaning "impelling cause or motive of any action" is from 1690s.

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mainstay (n.)

"chief support," 1787, a figurative use of a nautical noun meaning "stay which extends from the main-top to the foot of the foremast" (late 15c.), from main (adj.) + stay (n.).

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mainstream (n.)
also main-stream, main stream, "principal current of a river," 1660s, from main (adj.) + stream (n.); hence, "prevailing direction in opinion, popular taste, etc.," a figurative use first attested in Carlyle (1831). Mainstream media attested by 1980 in language of U.S. leftists critical of coverage of national affairs.
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maintain (v.)

c. 1300, maintenen, "to support, uphold, aid;" also "hold fast, keep in possession, preserve from capture or loss," from Anglo-French meintenir (Old French maintenir, 12c.) "keep (a wife), sustain; persevere in, practice continually," from Latin manu tenere "hold in the hand," from manu, ablative of manus "hand" (from PIE root *man- (2) "hand") + tenere "to hold," from PIE root *ten- "to stretch."

Sense of "hold in an existing state or condition, keep in existence or continuance" is from early 14c. Meaning "to carry on, keep up" is from mid-14c.; that of "to keep oneself, support" is from late 14c. Sense of "defend in speech, uphold by argument or assertion" is from mid-14c. Meaning "practice habitually" is from c. 1400. Sense of "furnish means for the subsistence or existence of" is from c. 1400. Related: Maintained; maintaining; maintains.

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maintainable (adj.)

"capable of being supported or upheld, sustainable, defensible," originally "admissible in law," mid-15c., from maintain + -able. Related: Maintainability.

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maintenance (n.)

mid-14c., maintenaunce, "wrongful interference in others' lawsuits by a lord or his followers," from Old French maintenance "upkeep; shelter, protection," from maintenir "to keep, sustain; persevere in" (see maintain). Meaning "action of upholding or keeping in good order" is from early 15c. That of "action of providing a person with the necessities of life," also "financial provision or support, that which maintains or supports" is from late 14c.

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main-top (n.)

also maintop, "top of the mainmast," late 15c.; see mainmast + top (n.1). By 1725 as "platform just below the head of the mainmast."

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