how did saying ‘shhhhhhh’ become the way to get ppl to be quiet?
Quite possibly related to the word hush.
When I had a new born kid they told us to shhh to calm our child. Was told it is similar to the ambient sound in the womb so babies are comforted by it. So maybe subconsciously some of that calming is in all of us and it makes us be quite.
The word is “hush” which means “quiet” — but over time the u and then the h got dropped.
However, in other cultures, people still use a hissing sound, so it probably has to do with it carrying well over short distances without drowning out background sounds.