1
general::
Phrase(s): off the air
not broadcasting (a radio or television program). • The radio audience won’t hear what you say when you’re off the air. • When the performers were off the air, the director told them how well they had done.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Not being broadcast, as in Once they knew they were off the air, the panelists burst out laughing. This idiom, along with the antonym on the air ("being broadcast"), dates from the 1920s, air being considered a medium for radio-wave transmission.
American Heritage Idioms