The word "sh" (or "shh") is an exclamation for silence:
Shh! They're listening...
Etymonline only mentions a date (1847) and the common practice of "putting a finger to the lips." Does anyone else have more information about its origin?
Many words which mean "silence, please" have the digraph 'sh'. E.g. hush and shush.
The origin of all these words however, was the Middle English word huisst(pronounced "wheesht"), which originated in round about 1350–1400A.D. Huisst as expected, meant "silence, peace".
The Scottish plea for silence also includes the 'sh' digraph: wheesht.
The sound for signalling a desire for quiet has long been associated with the sound 'sh'.
an answer for which I do not have a source, but is worth consideration, is that the sound "shhh" is soothing to babies, perhaps because it imitates the sounds in the womb.
It's been a while since I read the book, but Fredrik Lindström's "Jordens smartaste ord" (ISBN: 9789100580360) discusses the word "shh" in depth and posits it's the only word common to all human languages. It's even present in languages that don't use the sound.
Since it's been so long since I read the book, I don't know his sources.
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