plateau (n.)

1796, "elevated tract of relatively level land," from French plateau "table-land," from Old French platel (12c.) "flat piece of metal, wood, etc.," diminutive of plat "flat surface or thing," noun use of adjective plat "flat, stretched out" (12c.), perhaps from Vulgar Latin *plattus, from or modeled on Greek platys "flat, wide, broad" (from PIE root *plat- "to spread"). Meaning "stage at which no progress is apparent" is attested from 1897, originally in psychology of learning. In reference to sexual stimulation from 1960.

"enter a period of stability or stagnation, cease to rise," 1952, from plateau (n.). Related: Plateaued; plateauing.