1. When a (usually) young man masturbates or receives a hand job, and ejaculates all over his own shirt. He then has to self consciously walk around with a cum stain on his shirt, unless he was at home or had the foresight to bring an extra shirt with him.
2. Sloppily and shamelessly enjoying oneself without consideration for appearance or the feelings of others. Can be male or female
cum on the shirt (n) is another version of this saying
bob went to sally's house after school. she didn't want to blow him, so she tugged him off and he came on his shirt. Then he rode the bus home and felt hella awkward, since he didn't have a coat.
Suzy was being such a drunk ass at that party last night, she definitely came on her shirt.
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.