noun, plural stand·bys.
Origin of standby
Idioms and Phrases with on standby
Ready and waiting, as in We've got three more painters on standby. This expression originated in the navy in the 1940s, where it referred to someone being ready to come on duty as soon as required. From about 1960 it began to be widely used in aviation for a passenger waiting to take the first available seat on a full flight.