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"
A term used to tell your friends you have to take a shit when chicks are around so as to avoid embarrassement from the hot babes. 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....