Duke Kahanamoku (1890 – 1968) was a Hawaiian competition swimmer who popularized the sport of surfing. A Native Hawaiian, he was born to a minor noble family less than three years before the overthrow of the Hawaiian Kingdom. He lived to see the territory's admission as a state, and became a United States citizen. He was a five-time Olympic medalist in swimming, winning medals in 1912, 1920 and 1924.
John Wayne the actor comes to my mind when I hear the name Duke, but there is also a famous wrestler named Duke Myers. I think it is a manly name and fits a rough and tough character for sure. I love it and think it's a great nickname to use too.