friend muscle

To act like a tough guy when your friends are with you, to someone who you would not if you were alone.

Carlos and Jared: Did you talk any more shit to Bill when we were gone?
Xander: No, you guys werent here.
Carlos/Jared: But you're so tough, you have meaningless tattoos, you listen to rap music really loud, and you go by Xander instead of Alex or Alexander.
Xander: I guess we all just have friend muscle.