In a rowdy situation, there are many ways to ask people to be quiet. You can appeal to their sense of common decency; you can shout at them and make them jump, or you can raise a threatening finger to your lips while glaring like a retired army General whose golf course has been occupied, possibly by the Occupy Golf Course movement.
Each of these has their place, but what if you just want all sound to cease, with the minimum of fuss and bother? What if you need everyone just to settle down, straight away, because you can see trouble on the horizon? If you go steaming in there, all heavy handed, with a firm “SHUT UP!” there’s a chance you will be considered to be rude, and then they’ll be too busy bristling and thinking of a snappy comeback to listen to what you’ve got to say.
So, you need a phrase or two which can demonstrate authority without being aggressive, a phrase which has some dotty affection built into it, but still generates the result you want, which is the enshushment of all around you.
That phrase is pipe down (or possibly simmer down), possibly preceded by steady on if things are particularly raucous.
It comes from the navy in the 1800s, when sailing ships were controlled by blasts from the boatswain’s pipe (one of those nautical whistles). At the end of the day, he would pipe down the hammocks, a signal to go to bed, or if there was insubordination, the miscreant would be piped down below decks.
It’s only a small jump from that to telling people to pipe down, possibly first made by someone acting above their station and pretending to be an officer, in order to quell an argument or rowdy conversation. In any case, the term soon gathered a life of its own outside of the navy, and is still used to this day.
Pipe down has all of the authority of its military past, without being offensive or rude, so parents can use it when settling their children to sleep, and pub landlords can use it to try and prevent a fight. And because it’s such an odd phrase, one that carries no cultural baggage beyond hearing teachers say it at school, you can throw it around without worrying that you’ll wind up with a black eye.
And if it doesn’t work, you can always fall back on the tried and tested put a sock in it That’s slightly more aggressive, but it never fails.