Same as "raw dog" or to have sex without a condom. But its spelled backwards to make it more comedic and catchy. Just saying "God War" out loud is eneugh to get your buddys repeating it over and over.
Just like Little John and that whole "Yeahh" "OK" thing everyone shouted of years past.
"Dood Collins; GOD WAR!"
"Hell Yes, GOD WAR!"