level (n.) Look up level at Dictionary.com
mid-14c., "tool to indicate a horizontal line," from Old French livel "a level" (13c.), ultimately from Latin libella "a balance, level," diminutive of libra "balance, scale, unit of weight," from PIE *lithra. Cognate Spanish nivel, Modern French niveau are from the same source but altered by dissimilation. Meaning "horizontality" is from c.1400. Meaning "position as marked by a horizontal line" is from 1530s. Phrase on the level "fair, honest" is from 1872; earlier it meant "moderate, without great ambition" (1790).
level (v.) Look up level at Dictionary.com
mid-15c., "to make level," from level (n.). From c.1600 as "to bring to a level;" 1958 as "to cease increasing." Meaning "to aim a gun" is late 15c. Slang sense of "tell the truth" 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]
To level off "cease rising or falling" is from 1920, originally in aviation.
level (adj.) Look up level at Dictionary.com
early 15c., from level (n.). To do one's level best is from 1851.