from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Informal Having nothing due or owed on either side: an even-steven transaction.
- adj. Informal Having an equal score, as in a game or contest.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Equal; fair.
even1 + the name Steven, used as rhyming slang.(American Heritage® Dictionary of the English Language, Fourth Edition)
Possibly reduplicating rhyme from even ("equal"). See also steven ("money") (slang) (Wiktionary)