A saying meaning "it's a wasted effort" or "all is lost".
Possibly a reference to King John of England who lost his Crown Jewels and everything in his baggage train while attempting to take a shortcut across the Wash, which is an estuary in northeast England that floods. His baggage train was partway across it when the tide came in and lives and treasure were lost.
After waiting hours for their favorite celebrity to emerge from the backstage door, the girls gave up. "It's a wash; he's already gone out some other way."