means A surface or line that tilts, rising or falling at an angle rather than lying flat — or the steepness of that tilt.

from From Old English 'slupan,' to slip or slide, the same root that gives us 'sleeve' (something you slip your arm into) and 'slip' itself. The noun 'slope' broke off later, likely shortened from the older 'aslope,' meaning slanting or at an angle — so the word literally carries the sensation of sliding built into its bones.