Wanderlust World Wanderlust World In the original Hawaii Five-O (1968–1980), Detective Danny "Danno" Williams (played by James MacArthur) was the right-hand man to Steve McGarrett (Jack Lord). The nickname's origin was revealed in the series lore:
Steve McGarrett's grandfather—also named Steve McGarrett—was the original "Danno." The nickname was passed down through the family to the younger Steve (the series' protagonist). The show never explicitly explained why the grandfather earned the moniker.
In the CBS reboot, Danny Williams (Scott Caan) is a New Jersey transplant partnered with Steve McGarrett (Alex O'Loughlin). The "Danno" nickname is used affectionately/teasingly by McGarrett, but no multigenerational backstory is given.
"Book 'em, Danno" became one of TV's most famous catchphrases—McGarrett's sign-off after a case closed. The nickname itself, of uncertain etymology (possibly from "Daniel" + Hawaiian "-o" diminutive, or simply a family term of endearment), endures as shorthand for loyal partnership.