A request to be quiet or call for silence. On sailing the boatswain’s pipe was used as a tool to communicate with the ship’s crew. One such signal was ‘piping down the hammocks’ which was the signal to go below decks and retire for the night. When an officer wanted a sailor to be dismissed below he would have him ‘piped down’. Possibly also used to call sailors to silence so they could hear the commands of the officers.