catch a weasel asleep

To surprise someone who is normally alert, shrewd, or on-guard. Primarily heard in US. You want to try to steal your transcript from the principal's office? You never catch a weasel asleep—and especially not that one! Good luck catching a weasel asleep in this case. I mean, this is Mom we're talking about—she hears everything and will definitely hear you sneaking out of the house. I just don't see you pulling off this prank on Tim. He's wily, and it’s hard to catch a weasel asleep.