slope: [15] The noun slope did not emerge until the 17th century. Originally it was an adverb, short for the now defunct aslope. This is generally supposed to go back to an unrecorded Old English *āslopean, an adverbial use of the past participle of āslūpan ‘slip away’. Such a scenario would appear to fit in well with the colloquial slope off ‘leave’, but in fact this usage did not emerge until the early 19th century, in America.
slope (n.)
1610s, "inclination," from slope (v.). Meaning "an incline, a slant (of ground)" is from 1620s. Derogatory slang meaning "Oriental person" is attested from 1948.
slope (v.)
1590s, "go in an oblique direction," from earlier adjective meaning "slanting" (c. 1500), probably from Middle English aslope (adv.) "on the incline" (late 15c.), from Old English *aslopen, past participle of aslupan "to slip away," from a- "away" + slupan "to slip" (see sleeve). From 1709 as "to be in a slanting position;" transitive sense "place in a slanting position" is from c. 1600. Related: Sloped; sloping.