Telling someone to be quiet by saying 'shush' or making a sound like 'shhh.'
Onomatopoeia—the word 'shush' mimics the actual 'shh' sound people make to quiet others. It emerged in English around the 1840s as an informal way to represent this sound.
Librarians have been shushing people for so long that it became a stereotype, but the 'shh' sound actually works because it's a natural quieting signal that humans respond to almost instinctively!
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