"slope, slant," late 14c., first in Scottish writing and apparently meaning "edge, brink," a word of uncertain origin. "words identical in form and corresponding in sense are found in many languages, Teutonic, Slavonic, Romanic, Celtic" [OED]. It was rare in English before c. 1600. Meaning "slope, slanting or tilting position" is from 1847.
Perhaps via Old North French cant "corner" (itself perhaps via Middle Low German kante or Middle Dutch kant), from Vulgar Latin *canthus, from Latin cantus "iron tire of a wheel," which is possibly from a Celtic word meaning "rim of wheel, edge, brim" (compare Welsh cant "bordering of a circle, tire, edge," Breton cant "circle"). The ultimate connections of these are uncertain. Greek kanthos "corner of the eye," and Russian kutu "corner" sometimes are suggested, but there are difficulties (see Beekes).