Meaning : Put up your fists and get ready to fight.
This link claims that one can not be sure of origin of this phrase. Among three explanation given here, none make sense to me. S. Upendran writing in The Hindu has an explanation. Apparently, Duke of Wellington, the man who defeated Napoleon Bonaparte in the Battle of Waterloo in 1815, though quite the ladies man, wasn’t really much to look at. The most prominent feature of his not-so-handsome face was his rather long nose. The officers and the men who served beneath him began to affectionately refer to any nose which was unusually long as ‘duke’. With the passage of time, this word began to be used to refer to any nose; of whatever size. Since human fists are notorious to be employed in fights to put a ‘duke’ out of joints, fists began to be called ‘duke buster’. Soon, the word ‘buster’ was dropped, and everyone started referring to fists as ‘dukes’.
Thats a radical change in meaning!!