In short, this means the person "going on a t-shirt" is going to die. Then, when they die, their homies will wear t-shirts with the dead person's name on it in remembrance, therefore birthing the term "going on a t-shirt"
Person A: You not gon swing nigga!
Person B: Course I'm not gon swing, you going on a t-shirt
A term used to tell your friends you have to take a shit when chicks are around so as to avoid embarrassement from the hotbabes. Often followed by "the way's clear" to let your friends know you've gotten away with it.
Ben: Hey guys im going "on the watch" , (in whispers) Kyle make sure noones watching me on the watch.
Kyle:gotcha bro... Hey laura why dont we go over there....