Definition of safety-valve:
(v) :
(transitive) To use a relief valve on (a container holding a gas or vapour) to relieve excessive pressure which has built up inside.
(v) :
(transitive) To install a relief valve on (such a container) for that purpose.
(v) :
(transitive, figuratively) To use some mechanism to relieve (physical or emotional pressure or tension).
(v) :
Sometimes followed by off:
(n) :
Alternative form of safety valve. [A valve set to open at a pressure below that at which a container holding a gas, vapour, etc. (such as a boiler or pressure cooker), would burst, thus reducing the pressure; a relief valve.]