Centuries ago in middle Europe, a coded language appeared, scrawled in graffiti and spoken only by people who were “wiz” (in the know). This hybrid language, dubbed Rotwelsch, facilitated survival for people in flight—whether escaping persecution or just down on their luck. It was a language of the road associated with vagabonds, travelers, Jews, and thieves that blended words from Yiddish, Hebrew, German, Romani, Czech, and other European languages. Martin Puchner learned about this secret language from his father and uncle. Only as an adult did he discover, through a poisonous 1930s tract on Jewish names buried in the archives of Harvard’s Widener Library, that his own grandfather had been a committed Nazi who despised this “language of thieves.”