"slope," 1640s, from French talus (16c.), from Old French talu "slope, mound, small hill" (12c.), probably from Gallo-Roman *talutum, from Latin talutium "a slope or outcrop of rock debris," perhaps of Celtic origin (compare Welsh, Breton tal "forehead, brow").
OED, however, suggests derivation from root of talus (n.1) in the sense of "heel" which developed in its Romanic descendants. Mainly used of military earthwork at first; meaning "sloping mass of rocky fragments that has fallen from a cliff" is first recorded 1830.