It’s not often an old joke evolves into a catchphrase for mass violence, but that’s just what’s happened this past year when a variety of extremist and fringe movements and subcultures adopted the word “boogaloo” as shorthand for a future civil war. From militia groups to white supremacists, extremists on a range of online platforms talk about—and sometimes even anticipate—the “boogaloo.” The rise of “boogaloo,” and its casual acceptance of future mass violence, is disturbing. Among some extremists, it may even signify an increased willingness to engage in violence. “Boogaloo” has its roots in decades of jokes about an old...