From his IMBD trivia page: https://m.imdb.com/name/nm0456158/trivia

Jack Kirby was famous for being as tough as his characters. For instance, when a gangster was shaking down Kirby's boss, Will Eisner, to take his towel service, Kirby charged in to throw the gangster out. When Kirby created Captain America with Joe Simon, Nazi sympathizers once gathered in the lobby of the office building where the pair worked to threaten them. However, when upon hearing that Kirby was heading down with his sleeves rolled up to physically confront them himself, they fled before he arrived.