He may not have sworn on screen but I recall reading more than once how Wayne, Bond and Ford were way ahead of their time in swearing profusely in real life.
Duke according to Michael Munn: "Maureen is not like most other women. She didn't mind you using four-letter words around her, which I tend to do - often - and she didn't get all girly and dainty, and yet she is still totally feminine."
Munn also writes that he was having a lunch with Duke, all his grandchildren around. They were talking about Clint Eastwood and Duke disaproved the violence and profanity in his movies. Munn objected that there's actually very little bad language in Clint's films. Said Duke: "Bullshit! His films are full of f-ing obscenities!" None of his grandchildren paid any attention. They were used to his colourful language.