An announcement made to persuade crazy fans to quiet down. It was first made by the announcer of the popular television variety show, Louisiana Hayride, Horace Logan. It turns out that, even though Elvis was huge at the time, he was the third of about twenty acts that night. On December 15, 1956, he took the stage, and the fans squealed with delight, as always. After his 45-minute set and the ubiquitous encore, he left.
The fans didn't.
This was live TV in its heyday, but the shows were still scripted, and if you went over or under your allotted time, bits of the program--or worse yet, bits of the script that advertised the sponsor--got cut.
A desperate Horace Logan got on the mike, and vainly spoke to the screaming teeny-boppers: "Please, young people... Elvis has left the building... he has gotten in his car and driven away... Please take your seats..."
The phrase has earned itself a place in slang up there with "Thank you, thank you very much," and has been quoted in numerous movies including Independence Day.
The King is dead. Long live the King.