As a noun, "butthurt" means "an excessive or unjustifiable feeling of personal offense or resentment" (Oxford Languages). The term likely comes from the action of hurting one's butt on accident. If you slip and fall, it is likely you will fall and land on your butt. It is an unpleasant feeling and it hurts, but you will probably be able to stand up and brush yourself off as if it never happened. In other words, you will be okay. "butthurt" is used to describe when someone gets their feelings hurt or is offended by something that someone says or does. It can be used in a way that is toxic. "Let the butthurt flow through you" Let's say Adam has said something that Justin finds offensive. Adam knows that Justin will probably find it offensive, so he says this to let Adam know that he doesn't care about how Adam feels. Example: Adam: "I don't believe that people named Justin are smart. All of the Justins I have ever met were so dumb. That's just my opinion, though. Let the butthurt flow through you." (Maybe not the best example)

Haha. First, it's butt, not but. Butt is your rear end. But hurt is when someone gets offended /upset over something stupid. For example, offended over the book To Kill a Mocking. And the phrase "let the butt hurt flow through you" means to let" is a way to dismiss that person. Saying they can go ahead and rant, but you will not engage or pay attention to them, because they are wrong and not worth your time

to be butt hurt - to be profoundly bothered by something.