Cutting your shirt after your first solo?

What is the tradition behind that? Today I was hanging out with some instructors from my school, and one of them asked and nobody knew. I once heard that the tradition came because in the old times when the gun man sat in front of the pilot in old airplanes, and they didnt have no radio the pilot would tug on the gunman's shirt to get his attention since they didnt have radios o intercoms they had diffculty talking to each other. Thats what I heard does somebody know?