The origin of this expression comes from 6 pm news casts when a ‘70s
newscaster would report on a story but urge viewers to tune in for the 11 pm newscast for footage of the event. There was no footage because this was before portable video cameras (aka
ENG). If a newscast wanted to show moving images, they had to send out a film crew with a 35 mm film camera, film, and then take it back to the studio, develop the film, and edit the film together was
razor blades and
editing tape.
To expand upon the illustrious
Jargon File’s definition, the expression just generally means “hang on for a bit for some astounding news.” It can be used sarcastically or to actually convey the notion that one should be patient for some real news.