A loop in the back of your shirt can mean different things
This piece of information is definitely not useful for you if you have a garden-variety shirt. On the other hand, if you wear a collared shirt, the back must have a loop. You can thank the Navy for it since sailors’ shirts had loops on the back so they could conveniently hang them on hooks that did not have much storage space available for their uniforms.
Later on, some students repurposed the loop to convey their relationship status. Apparently, it was assumed that if a man’s loop was missing, he was dating someone and was not available. Moreover, they were fashioned to keep students’ shirts wrinkle-free in Ivy League locker rooms. Nowadays, you can still find the loop on some shirts.