Etymology
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level (adj.)
early 15c., "having an even surface," from level (n.). Meanings "lying on or constituting a horizontal surface" and "lying in the same horizontal plane" (as something else) are from 1550s. To do one's level best is U.S. slang from 1851, from level in the sense "well-aimed, direct, straight." Level playing field as a figure of equality of opportunity is from 1981. Related: Levelly.
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level (v.)

mid-15c., "to make level" (transitive), from level (n.). From c. 1600 as "to bring to a level." Intransitive sense "cease increasing" is from 1958. Meaning "to aim (a gun)" is late 15c. Slang sense of "tell the truth, be honest" is from 1920. To level up "to rise" is attested by 1863.

A word here as to the misconception labored under by our English neighbor; he evidently does not understand the American manner of doing things. We never level down in this country; we are always at work on the up grade. "Level up! Level up!" is the motto of the American people. [James E. Garretson, "Professional Education," in "The Dental Cosmos," Philadelphia, 1865]

Modern use is mostly from computer gaming (2001). To level off "cease rising or falling" is from 1920, originally in aviation. Related: Leveled; leveling.

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level (n.)

mid-14c., "tool to indicate a horizontal line," from Old French livel "a level" (13c.), ultimately from Latin libella "a balance, level" (also a monetary unit), diminutive of libra "balance, scale, unit of weight" (see Libra). Spanish nivel, Modern French niveau are from the same source but altered by dissimilation.

Meaning "position as marked by a horizontal line" (as in sea-level) is from 1530s; meaning "flat surface" is from 1630s; meaning "level tract of land" is from 1620s. Figurative meaning in reference to social, moral, or intellectual condition is from c. 1600. Figurative phrase on the level "fair, honest" is from 1872; earlier it meant "moderate, without great ambition" (1790).

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split-level (adj.)
1951 as a type of building plan, from split (adj.) + level (n.). As a noun from 1954, short for split-level house, etc.
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level-headed (adj.)
also levelheaded, "sensible, shrewd," 1869, from level (adj.) + -headed. The notion is of "mentally balanced." Related: Levelheadedness.
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multilevel (adj.)

also multi-level, "having or involving several levels," 1952, from multi- "many" + level (n.).

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leveller (n.)
also leveler, 1590s, someone or something that levels or makes even; agent noun from level (v.). In English history, from 1640s (with initial capital) as the name of a political party of the time of Charles I that advocated abolishing all differences of position and rank.
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plain (n.)

"level country, expanse of level or nearly level ground," c. 1300 (in reference to Salisbury Plain), from Old French plain "open countryside," from Latin planum "level ground, plain," noun use of neuter of planus (adj.) "flat, even, level" (from PIE root *pele- (2) "flat; to spread"). Latin planum was used for "level ground" but much more common was campus.

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flats (n.)
"level tidal tract," 1540s, from flat (n.) in the Middle English "level piece of ground" sense.
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plani- 

word-forming element meaning "level, flat, plane," from Latin plani-, from planus "flat, level" (from PIE root *pele- (2) "flat; to spread").

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