put up one's dukes

Possibly by analogy to a king or other ruler summoning his dukes, and by extension the duke's knights or other soldiers, to battle an enemy. Another possibility is Cockney rhyming slang as explained at duke. It could also be a convoluted--though erroneous--reference to the 9th Marquess of Queensbury (after whom the rules for modern boxing were initially named).

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put up one's dukes (third-person singular simple present puts up one's dukes, present participle putting up one's dukes, simple past and past participle put up one's dukes)

1. (idiomatic) To raise one's clenched fists in front of one's body and stand in a threatening or defiant manner, in preparation for a fistfight.
   • 1886, Edward Money, The Truth about America‎^[1], page 206:

     The proceedings of the State Democratic Convention, held at Turner Hall, yesterday, were disgraceful enough to bring a blush even to the cheek of a Democrat. "Liar," "snide," "put up your dukes, if you want to fight," catcalls, hooting, and yelling filled up a greater part of the deliberations of the august body.

2. (idiomatic, by extension) To take firm action or to show oneself to be committed to such action, as when competing in a sporting event or other contest.
   • 2008, Jonathan Schaeffer, One Jump Ahead: Computer perfection at checkers‎^[2], →ISBN, page 401:

     The NBC reporter who came to the match on Wednesday told Lafferty "Come on, put up your dukes, we want to see some blood."

• duke it out