I am a man of many insults. Which is why I've always struggled buying a t-shirt with an insult on it. There is no one insult which is a true one size fits all, different insults for different people and different times. For example, calling your brother a twatfink as he falls down the stairs is fine, but this may not be considered acceptable at a posh ball (not that I ever attend anything so pretentious).
SO! I present this shirt to you! With a finger points straight to the viewer, the poor sod infront of you can fill in the blank, leaving you to insult them at will without being bound by your clothing. You could even turn it into a game, asking everyone who passes you by what their insult of choice was. You could then insult them in return for being wrong, or for being right, when they guess what yours was.