This phrase is declared when an electrical socket is needed for personal usage (i.e. a phone charger), but a limited number of sockets are available for the number of people present (i.e. a family). Thus, the socket is "reserved". Stems from the phrase, shotgun, which is declared to reserve the front passenger seat of a car.
My phone needs charging, and there's only one socket. Shot plug!
by G. Thomas June 24, 2008