When someone tries to act all big and cool on the internet by posting offensive and rude posts and comments on forums, youtube, etc. Often done to attempt to feel good about one's sad pathetic lonely life.
Nolifer: "you fucking idiot dont know anything about this go to fucking hell lolol i fuck your mom bitch!!
Human Being: "Stop flexing your internet muscles, everyone knows you aint shit"
The same as "beer muscles", where when a person is drunk, they do and say things they wouldn't normally have to courage to do, or say. Internet muscles are where a person gets online, reinvents themselves, and says things they can't say in public, like using racial slurs for example.