When you are about to beat the living crap out of someone and you tell them to "Go Get God" because the beating is inevitable and they need some higher power to live through it.
An encouragement used in ordinary conversation since the 1960's. Expressing your confidence someone has qualities on par with a jungle cat: stealthy and lethal. "Slugger" was a similar boost that didn't survive modernity, now considered mildly sardonic rather than sincere.