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.