Created by the legendary Funky_Del as a more fitting replacement for lol; means "That made me lick my balls!". To be more precise, lol in messaging services nowadays is more of a general part of a sentence, like a full stop or comma, than to mean you actually "laughed out loud", and in contrast "lmao" seems too much to use for things that are only vaguely funny. Therefore, tmmlmb was created.
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.