fail-safe adj.

c. 1200, "be unsuccessful in accomplishing a purpose;" also "cease to exist or to function, come to an end;" early 13c. as "fail in expectation or performance," from Old French falir "be lacking, miss, not succeed; run out, come to an end; err, make a mistake; be dying; let down, disappoint" (11c., Modern French faillir), from Vulgar Latin *fallire, from Latin fallere "to trip, cause to fall;" figuratively "to deceive, trick, dupe, cheat, elude; fail, be lacking or defective." De Vaan traces this to a PIE root meaning "to stumble" (source also of Sanskrit skhalate "to stumble, fail;" Middle Persian škarwidan "to stumble, stagger;" Greek sphallein "to bring or throw down," sphallomai "to fall;" Armenian sxalem "to stumble, fail"). If so, the Latin sense is a metaphorical shift from "stumble" to "deceive." Related: Failed; failing.

Replaced Old English abreoðan. From c. 1200 as "be unsuccessful in accomplishing a purpose;" also "cease to exist or to function, come to an end;" early 13c. as "fail in expectation or performance."

From mid-13c. of food, goods, etc., "to run short in supply, be used up;" from c. 1300 of crops, seeds, land. From c. 1300 of strength, spirits, courage, etc., "suffer loss of vigor; grow feeble;" from mid-14c. of persons. From late 14c. of material objects, "break down, go to pieces."

c. 1300, sauf, "unscathed, unhurt, uninjured; free from danger or molestation, in safety, secure; saved spiritually, redeemed, not damned;" from Old French sauf "protected, watched-over; assured of salvation," from Latin salvus "uninjured, in good health, safe," which is related to salus "good health," saluber "healthful" (all from PIE *solwos from root *sol- "whole, well-kept"). For the phonological development of safe from sauf, OED compares gage from Old North French gauge.

From late 14c. as "rescued, delivered; protected; left alive, unkilled." The meaning "not exposed to danger" (of places, later of valuables) is attested from late 14c.; in reference to actions, etc., the meaning "free from risk," is recorded by 1580s. The sense of "sure, reliable, not a danger" is from c. 1600. The sense of "conservative, cautious" is from 1823. It has been paired alliteratively with sound (adj.) from c. 1300. In Middle English it also meant "in good health," also "delivered from sin or damnation." Related: Safeness.