1520s, "to strike obliquely" (against something), alteration of slenten "slip sideways" (c. 1300), perhaps via a Scandinavian source (compare Swedish slinta "to slip," Norwegian slenta "to fall on one side"), from Proto-Germanic *slintanan. Intransitive sense of "to slope, to lie obliquely" is first recorded 1690s; transitive sense of "to give a sloping direction to" is from 1805. Related: Slanted; slanting. As an adverb from late 15c.; as an adjective from 1610s. Slant rhyme attested from 1944.
1650s, "an oblique direction or plane" (originally of landforms), from slant (v.). Meaning "a way of regarding something" is from 1905. Derogatory slang sense of "a slant-eyed Asian person" is recorded from 1943, from earlier slant-eyes (1929).