key·board war·ri·or
nounINFORMAL
noun: keyboard warrior; plural noun: keyboard warriors
a person who makes abusive or aggressive posts on the internet, typically one who conceals their true identity.

Keyboard Warrior
1. A Person who, being unable to express his anger through physical violence (owning to their physical weakness, lack of bravery and/or conviction in real life), instead manifests said emotions through the text-based medium of the internet, usually in the form of aggressive writing that the Keyboard Warrior would not (for reasons previously mentioned) be able to give form to in real life.