What does ‘alarm clock’ mean?

The sound of an alarm clock is usually met with moans and groans; the same reaction after learning there’s going to be a chaperon at a dance or on a date.

Alarm clocks, a slang term for “chaperon,” at the dance in the 1920s meant you couldn’t neck on the dance floor … or pet each other in the petting pantry (which is the movie theater, of course).