Rager is an adjective used to describe someone who deserves to be hated by everyone. It's derived from the term raging.. As in someone could be a ragin pedophile or a raging fag or even a raging nig-nog. The term rager is used on anyone who you feel fits the description of all of these insults.
Father John O'reilly is a rager.
If you play counterstrike you are a rager.