1. Electronics. pertaining to or noting a mechanism built into a system, as in an early warning system or a nuclear reactor, for insuring safety should the system fail to operate properly.
  2. equipped with a secondary system that insures continued operation even if the primary system fails.
  3. (sometimes initial capital letter) of, relating to, or designating a system of coded military controls in which bombers dispatched to a prearranged point as part of a standard operating procedure cannot advance farther without direct orders from a designated authority and cannot have the nuclear warheads they carry armed until they have passed their prearranged point.
  4. guaranteed to work; totally reliable: a fail-safe recipe for a cheese soufflé.


  1. (sometimes initial capital letter) the point beyond which the bombers cannot go without specific instruction; the fail-safe point.
  2. something designed to work or function automatically to prevent breakdown of a mechanism, system, or the like.

verb (used with object), fail-safed, fail-saf·ing.

  1. to make fail-safe.


  1. designed to return to a safe condition in the event of a failure or malfunction
  2. (of a nuclear weapon) capable of being deactivated in the event of a failure or accident
  3. unlikely to fail; foolproof


  1. (intr) to return to a safe condition in the event of a failure or malfunction

also fail-safe, fail safe “safe against failure,” 1948, from fail (v.) + safe (adj.). Earliest reference is to furnace burners.

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