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.
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).
"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.