level (v.)youdaoicibaDictYouDict
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.
plane (v.1)youdaoicibaDictYouDict
"to make smooth," early 14c., "to gloss over, explain away;" mid-14c. as "to make smooth or even," from Old French planer "to smooth, level off; wipe away, erase" (12c.), from Late Latin planare "make level," from Latin planus "level, flat" (see plane (n.1)). In early use in English often plain. Related: Planed; planing.