be as good as gold

1. To be obedient and well-behaved. Sure, I'll baby-sit Cindy's kids tonight—they're as good as gold every time I watch them. My students are always as good as gold, until they get some sugar in them at lunch. A lot of teenagers rebel, but not me—I was always good as gold.

2. To be authentic and legitimate. According to the curator, this painting is as good as gold. Dude, we'll be rich if these diamonds prove to be as good as gold! Everyone thinks they've uncovered untold riches in their attic or basement. It's up to our appraisers to determine what's really good as gold.