fail-safe adjective 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, 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. noun 5. ( sometimes initial capital letter ) the point beyond which the bombers cannot go without specific instruction; the fail-safe point. 6. something designed to work or function… Read moreOperation Fail-Safe the Restrainer