an abbrievation of sucks to be you. this is used when someone else has an fml moment and you really dont feel sorry for them. you actually find it funny, and are glad that it didnt happen to you. stbthem or stbhim/her can also be used.
bob: today i asked a girl out and she rejected me and then i ran into a locker and broke my nose.
jimmy: stbu!! hahahahha
jimmy and bob drive by the school band practicing in the pouring rain.