1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): on standby
waiting for one’s turn, especially describing the status of travelers who wait at a soon-to-depart train, plane, or bus, hoping that a seat will become available. • The passenger waited on standby for an available seat. • The agent was able to seat all of the passengers on standby.
McGrawhill's American Idioms And Phrasal Verbs