c. 1300, restoren, "to give back," also, "to build up again, repair; renew, re-establish; free from the effects of sin; bring back to a former and better state," from Old French restorer, from Latin restaurare "repair, rebuild, renew." This is from re- "back, again" (see re-) + -staurare, not attested by itself but also in instaurare "to set up, establish; renew, restore," etc., from PIE root *sta- "to stand, make or be firm."
From late 14c. as "to cure, heal, bring back to a vigorous state;" of objects, beliefs, etc., "bring back to an original state or condition," 1670s. Related: Restored; restoring.