This means basically the same thing as “even” — having no balance of debt on either side; even in the setting of accounts, having an equal chance or score; tied. This colloquial phrase is a colorful way to talk about whether a deal or scenario is fair. And it’s more fun to use rhyming slang than a straightforward simple word.
Linguists are interested in the effect of rhyming. Naturally, different people perceive rhyming slang differently, but apparently the general effect is to emphasize the meaning. For example, saying “even-steven” may cause the listener to focus more on fairness than if the speaker just said “are we even?” For some people, the use of rhyme also lightens the communication, because it’s a more playful description.
Brits and Aussies use the term “even-stevens.” The phrase has been around since at least the mid-nineteenth century. One theory of the origin of is a line in Journal to Stella by Jonathan Swift: “‘Now we are even,’ quoth Steven, when he gave his wife six blows to one.” Another theory is that steven or stephen was a British slang term for money, according to A Dictionary of Slang and Colloquial English by Farmer and Henley. Even-steven and even-stevens are most often hyphenated, but steven and stevens are never capitalized.
Some say that the phrase “even-stevens” became much more popular in the 1960s after a New Zealand race horse called “Even Stevens” won several prominent races. The phrase has also been traced back to its use in various books and periodicals. Another source says the horse was Australian.
Here in Canada, I’ve always heard the phrase as the singular “even-steven.” I haven’t often heard it used, in fact, but rhyming slang has always been a lot more popular in Britain than in North America.
Wherever it came from, I’m glad it’s here and in use. I much prefer colorful language to formal.