BBB Etymology - Pipe down

The pipe in question was the boatswain's pipe, a small whistle-like device used by the boatswain (the petty officer in charge of the deck) to communicate orders to the crew via different arrangements of notes. When the boatswain "piped down" aboard ship, he blew the signal for the crew to retire from their tasks or formation and return to their quarters belowdecks. Since the deck would become suddenly quiet when the crew retired, "pipe down" came to be used as nautical slang for "be quiet" or "shut up," and by the end of the 19th century it had percolated out into its modern non-seafaring usage.