OK, now we can begin.
I’m not sure what got me thinking about this word (or, ejaculation at least, I’m not sure it merits being considered a word). It’s certainly not one I like to use much. Still, it’s curious.
Does it come from an original word? Some believe it’s derived from the Middle English huisst, used a a command for silence. A variant, whist or whisht, is still used occasionally in Hiberno-English, though it’s now pretty old fashioned. The most effective, emphatic part is the sh sound, so it’s a pretty convincing case as the origin of sh.
But then, how come it’s so used across so many languages and cultures? It’s unlikely that a word from Middle English would have spread so much before the era of mass communication and convenient worldwide travel.
Perhaps it’s a case of sound symbolism, with there being something inherent to the sound of sh that communicates the idea of silence.
Or, perhaps it’s just convenient. For such a seemingly simple sound, it can be used in a variety of ways. If you want to use it quietly (and isn’t it often the case that you need to shush someone in a quiet environment?), it’s quite easy to say shh at a low volume, but with the emphasis on the sh still audible for your target. It can be soothing (or patronising), or loud and harsh.
Maybe we used many different sounds over time, but we all independently converged on sh because it works much better than other sounds due to its versatility.