The throbbing, oversized lips of a STD infected mouth. Usually received from putting your mouth on a muffin dick or a vagina muffin (see my definitions for these). They hurt like a bitch and people won't understand what the fuck you're saying. Muffinlips cannot be cured.
"Hey dude I licked my girls vadge and now my lips hurt" "I can't understand you but fuck I think you have muffinlips."
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.