Wikipedia has a good description of the tradition of cutting a pilot’s shit tail after completing his first solo:
“In American aviation lore, the traditional removal of a new pilot’s shirt tail is a sign of the instructor’s new confidence in their student after successful completion of the first solo flight. In the days of tandem trainers, the student sat in the front seat, with the instructor behind. As there were often no radios in these early days of aviation, the instructor would tug on the student pilot’s shirttail to get his attention, and then yell in his ear. A successful first solo flight is an indication that the student can fly without the instructor (“instructor-less” flight). Hence, there is no longer a need for the shirt tail, and it is cut off by the (often) proud instructor, and sometimes displayed as a trophy.”