Money SHE doesn't know you have. She being the wife, the girlfriend, the roommate, etc. Money kept secret for spending on hidden purposes, such as buying things on the sly, strip club outings, gambling, drinking, investing, or planning to dump her and set up a new place.
Dave: "I got a hundred fifty from her - I had three fifty saved up in my she money account at the bank!"
Diane: "Honey, where'd you get the money for those Jimmy Choos?"
Stephanie: "Found my husband's She-Money box in the closet."