Takefumi Auchi is a prosecutor infamous among defense attorneys for his "Rookie Killer" reputation—he's made a habit of destroying inexperienced lawyers in court. He's best known as Ryuuichi Naruhodou's first opponent, which turned out to be way more significant than anyone expected at the time.
According to Auchi himself, he never lost a single case during his first seven years as a prosecutor. That's a pretty bold claim, but it does explain the nickname.
He and Naruhodou faced off in a murder trial, with Naruhodou fresh out of law school and basically green as they come. Auchi, naturally, had an easy time of it—and he's never let Naruhodou forget it. He's made some cheekily boastful remarks over the years about how he basically taught the guy everything he knows, which says something about Auchi's personality if nothing else.
Content compiled by AnimeList.moe from publicly available sources.
