it universal?

Yus, I understand that ‘shhhhh’ comes from hush, but I was more interested in the finger to the mouth (held crossways to the lips).

Does anybody come from a culture/language group where this is not performed as a gesture to keep quiet?

Well, it caught on better than ‘YI! YI! YIYIYIYIYI!!!’

I do know that sibilants are often used for this, as they are easily recognized at low volumes. I don’t know anything about the gesture, but I would not guess it was universal, simply because it doesn’t make a lot of sense. It’s too complicated to be evolutionary, so it must be cultural.

In some arabic cultures, the “sshhh” sound is rude and unacceptable.
It’s a sound that camels react to, and is said by the rider as he pulls hard on the reins and forces the camel to kneel.
You do not say “shhh” to a person.

What do they say when you are hiding in a cupboard and an axe murderer is approaching?

And is it the same sound as hushing and soothing a crying child?

I think in all languages I know it’s the same sound, and all shh- or ssst-like.