fail-safe

UK: /ˈfeɪlseɪf/ US: (fāl′sāf′)

Inflections of 'fail-safe' (v): (⇒ conjugate)

fail-safes

v 3rd person singular

fail-safing

v pres p

fail-safed

v past

fail-safed

v past p

WordReference Random House Unabridged Dictionary of American English © 2026

fail-safe  (fāl′sāf′), adj., n., v., -safed, -saf•ing. 
adj. 

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. Electronics equipped with a secondary system that insures continued operation even if the primary system fails.
3. Military (sometimes cap.) of, pertaining 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.

n. 

5. Military (sometimes cap.) the point beyond which the bombers cannot go without specific instruction;
   the fail-safe point.
6. something designed to work or function automatically to prevent breakdown of a mechanism, system, or the like.

v.t. 

7. to make fail-safe.

• apparently from verb, verbal phrase to fail safe(ly) 1945–50

Collins Concise English Dictionary © HarperCollins Publishers::

fail-safe adjective

1. designed to return to a safe condition in the event of a failure or malfunction
2. unlikely to fail; foolproof