When you say ‘Pipe Down’ you mean “be quiet!”
Example of use: “Tell the kids to pipe down. I can’t hear the TV!”
The origin of the expression ‘pipe down’ is nautical. Whistled signals are given to crews via a boatswain’s pipe, with one of those signals meaning “time to go below decks and retire for the evening,” or “be quiet as it’s time for bed.” This has been used since the late 16th century.