Indeed, mum is a word, meaning (in but one of its four noun-incarnations) "an inarticulate sound made with the lips closed," according to the OED online, and--this is lovely--the earliest reference is to Piers Plowman in 1400. I can imagine Piers making that sound a lot.
Oh, I thought I was in the vicinity of clever when I decided to write a poem envisaging a club devoted to quietness and playing off the phrase "mum's the word," but then (I should have known) I found out that the quirky 18th century got there way ahead of me. The OED online cites one of Joseph Addison's essays as referring to . . .
1711 J. ADDISON Spectator No. 9 ¶6 The Mum Club (as I am informed) is an Institution of the same Nature, and as great an Enemy to Noise.
It was a great age of clubs and--the Mum Club notwithstanding--conversation: the exuberant 18th century in London.
Meanwhile, our own era seems to be a great Friend to Noise. Alas and alack. Here's the poem:
Mum Is The Word
The League of Quiet Persons meets
monthly. Its quarters are a cavernous
warehouse away from traffic. Its
business is not to discuss business.
Minutes are read silently and tacitly approved.
Members listen to rain argue with corrugated
iron, a furnace with itself. Glances
are learnéd. It is not so much refuge
from noise the members seek in such company
as implicit permission not to speak,
not to answer or to answer for,
not to pose, chat, persuade, or hold forth.
Podium and gavel have been banned,
indeed are viewed as weaponry.
A microphone? The horror.
Several Quiet Persons interviewed
had no comment. A recorded voice
at the main office murmured only, “You
have reached the League of Quiet
Persons. After the tone, listen.”