Put up your dukes
Put up your fists and prepare to fight.
The dukes are the hands or fists. Before we get to put up your dukes, we need an explanation of how that came to be so. That use of dukes meaning hands is first referred to in print in the mid 19th century. George Washington Matsell records the link in Vocabulum; or, The rogue's lexicon, compiled from the most authentic sources, 1859:
[Pugilists' slang] Dukes. The hands.
Samuel E. Chamberlain's My Confession, circa 1859, also makes the link between dukes and hands/fists.
I landed a stinger on his "potatoe trap" with my left "duke," drawing the "Claret" and "sending him to grass."
No one is certain how dukes came to mean fists. There are three commonly repeated suggestions:
1. It derives from the Cockney rhyming slang - Duke of Yorks = forks = fingers. At first sight this seems rather unlikely. The earliest citation of Duke of York as rhyming slang (John C. Hotten, 1874) lists it as meaning walk. Also, the link between forks and fingers is hardly intuitive. There is a clear link though - forks had been a slang term for fingers/hands for many years by 1859. It is recorded as slang for pickpocket in Nathan Bailey's, An universal etymological English dictionary, 1737:
"FORK, a Pick-pocket. Lets Fork him; Let us pick that Man's Pocket. It is done by thrusting the Fingers, strait, stiff, open and very quick into the Pocket, and so closing them, hook what can be held between them."
James Hardy Vaux records it as slang for fingers in A new and comprehensive vocabulary of the flash language, 1812:
"Forks, the two fore-fingers; to put your forks down, is to pick a pocket."
The terms fork-out (or over or up), meaning pay money come from this same source and are recorded by 1831.
2. It is suggested to be of Romany origin. This belief comes from the Romany word dookin, meaning fortune telling or palmistry. H. Brandon, the editor of Poverty, mendicity and crime... To which is added a dictionary of the flash or cant language, 1839, lists this meaning in the book's glossary:
"Dookin - fortune telling."
The palmistry association does link dooking with hands, but, that aside, the evidence to support the Romany source isn't very strong.
3. It derives from an association with the Marquis of Queensberry, who endorsed the well-known Queensberry Rules for the sport of boxing. This source can be discounted as - a: he wasn't a duke and b: the Rules weren't published until 1865.
Of these three options, the first appears the most likely, although without further evidence that is conjecture.
What we do know is that put up your dukes was known by 1874 - the first record of it that I can find in print is in John C. Hotten's 1874 edition of A dictionary of modern slang, cant, and vulgar words:
"Dooks, or dukes, the hands, originally modification of the rhyming slang 'Duke of Yorks,' forks = fingers, hands... The word is in very common use among low folk. 'Put up your dooks' is a kind of invitation to fight."
Put up your dukes is a predecessor of the US phrase to duke it out, meaning to fight. This is a relatively late variant and, although the term duke, meaning to fight - specifically boxing, had been common in the USA for some time before, 'duke it out' didn't occur until the 1960s; for example, Garry Niver, in the The San Mateo Times, May 1964:
"As expected, the Mid-Peninsula League baseball chase is going right down to the wire. However, instead of two teams figuring to duke it out for the coveted crown, there will be three."
The figurative use of a contest not involving fisticuffs suggests that 'duke it out' was coined earlier than the above example. It can't have been more than a few years earlier though.