The expression “even steven” means with no advantage to either; as, to swap knives “even steven.”
Sometimes written with a capital, “Steven,” and sometimes appearing as “Stephen.”
We’ve had the phrase as colloquial American for at least a hundred years, in print since 1866, but that is about all one can say of it.
It is quite likely that it is nothing more than one of the numerous rhythmical reiteratives in the language, such as dilly-dally, shilly-shally, hodgepodge, ‘ods bods, hocus-pocus, ding dong, hell’s bells, and so on.
