A Bruce, is a friend(s) that makes up excuses not to go have a boys night out.
You're out with yourboys for a boys night out. One of yourboys turns to you and says he has to leave. You turn to him and ask why. He then says he has to go chill with his girlfriend or just doesn't feel like having a late night out... If it's before 11 PM, he is being a BRUCE ! When he insists on leaving you simply say " Don't Be A Bruce "