To shut-up or be quiet.
This idiom is used in the context when one creates a hell of nuisance or disturbances.This is also applied when one talks something rubbish and is asked to shut up.
1)My mother asked me to pipe down in the party when I was literally creating disturbances to the guests.
2)I asked my friend to pipe down when he started blabbering.
3)Robert ordered Peter to pipe down when he was talking rubbish in a very important meeting.
4)My friend requested me to pipe down as his father was meditating.
5)"Pipe down or else get out of my class", shouted the teacher at the student.