keyboard warrior; Internet tough guy
Colloquial term for someone who acts tough or aggressive online but is meek in person. Derived from 内弁慶 (a person who is only bold at home).
He's a keyboard warrior; in real life he's very meek.
Let's behave politely online too, so we don't become Internet tough guys.
Compound of ネット (Internet) and 弁慶 (Benkei, a famously strong warrior monk), modeled after 内弁慶 (a person who is only bold at home).