Johnson was a soldier in the Enclave. He served in Captain Judah Kreger's squad at Navarro with Orion Moreno, Doctor Henry, Daisy Whitman, and Gannon Senior; he was known to often be in conflict with fellow squad-mate Orion over "philosophical" differences - as in, Moreno was ruthless and Johnson had reservations about killing. Johnson hated the Enclave and everything they did; he often tried to subvert orders and "serve without serving."
Some time after the destruction of the Enclave Oil Rig, the NCR attacked Navarro and he, the squad, a young Arcade Gannon's mother, and Arcade himself fled south into the NCR. They continued to flee east as Enclave personnel trying to integrate into the NCR were being discovered. They eventually ended up in the Mojave Wasteland, and went their separate ways. When asked about his nickname, he explains to the Courier that one day when he was surrounded by raiders, he did the first thing that occurred to him: bite into the heart of one the raiders he had taken down. This shock tactic caused some of the others to flee and bought him enough time to kill the rest. Since then, the name has stuck, his improvisation growing into mystique. He now sits alone in his cave, waiting for death to take him for the sins he had committed while with the Enclave. He also reminisces about his former commander Arch Dornan, saying that "he was the meanest bastard I ever knew."
If you continue to talk to him about the Enclave, he will have a dialogue option about a drill sergeant who once caught a soldier without his uniform. This is a reference to Fallout 2 when the Chosen One went to Navarro and talked to the drill sergeant there. When the Chosen One goes and talks with the drill sergeant, the drill sergeant will yell at him for being out of his uniform.