Pseudo-French for "slut of the day"- the person, of the many indistinguishable options, with whom one is currently romantically engaged.
"Who's the girl Nick's with tonight?"
"Oh who knows, just his slut du jour, he'll have another by friday."
"...and the 'slut du jour' is Keith, who doesn't much care who's mouth he's in. Served with a side of Carlos."