Cruel and unusual
Anyone who comes across this cylindrical object with soft spikes on its exterior would think that it would be used for… things. Well, that’s definitely the case, but the question is what sort of things? The non-Korean-speaking OP had no idea how to find out the purpose of this soft piece of plastic.
According to more learned folks on the internet, this is a hand massager. It works by rolling it gently in your palms to reduce muscle tension. Others thought that it would also be great for… things. What sort of things? We’ll leave that to your imaginations.