early 15c., restif, restyffe, of animals, "not moving forward," from Old French restif "motionless, brought to a standstill" (Modern French rétif), from rester "to remain" (see rest (v.2)).
Rare or archaic in the original sense; the prevailing meaning "refusing to stand still" especially of horses (attested by 1680s) probably is based on the notion of "unmanageable, impatient in restraint" in reference to a horse refusing to go forward (1650s).
But it also is perhaps influenced by rest (v.), an old aphetic form of arrest "to stop, check," and by confusion with restless. Compare resty in the same sense, 1510s of horses, c. 1600 of persons. Related: Restively; restiveness.