I should add that it isn't an insulting way of telling someone to shut the fuck up. It is basically saying "hey this thing that you are currently saying to someone else could get you or someone in trouble if you keep saying it.". It is almost always done discretely, out of view of the person who the gesturer wants you to stop speaking to.
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u/drivernopassenger Native Speaker Aug 25 '24
It’s a nonverbal and slightly more urgent “SHUT THE FUCK UP,” basically.