Once upon a time not so long ago, in a land not so far away, there lived a teenage girl who loved playing fast pitch softball. It's important that we understand a few things about this girl. She always tried to do as her coach said (This made her VERY unpopular with some of the younger players.), she was not a girly-girl (more of a tomboy), and she tried to avoid conflicts.
One day as this girl and her teammates were going from the batting cages to the gym, they encountered a body lying on the steps leading up to the entry by the gym. One of the teammates pointed it out and another teammate screeched, "Ooh! What is it?!". The girl whom this tale is about replied in an incredulous, matter of fact way, "It's a mouse." After all, if it bothered the teammate that much, then she could just walk around it. Right? For some reason this bothered the screeching teammate (who I might add was popular) and she took it upon herself to get back at the girl for bothering her.
The teammates began whispering "mouse" under their breath any time they passed the girl. The girl, peace-keeper that she was, tried to ignore her teammates. It was annoying and she was somewhat hurt, but she kept it to herself. Soon, the teammates got braver and began calling out "mouse" any time it was the girl's turn in a drill and when the girl was up to bat. This practice carried on to the point that even the coach, who didn't know the origin, called the girl "Mouse". It then carried over to the games. Soon everyone, including the girl's family, called her "Mouse".
Now you know the true story about how a teenage girl came to be known as Mouse.