As Merriam-Webster puts it [1]

You will have seen thousands of them, because they are the conventional way to indicate a cartoon character is swearing, like this




There is even an emoji

🤬

I've often wondered about them but until this evening had no idea they has a special name. Now I am wiser, thanks to the latest Words Unravelled podcast.

[1] https://www.merriam-webster.com/wordplay/grawlix-symbols-swearing-comic-strips