(one's)/the cheese slid off (one's)/the cracker

dated One has become crazy, insensible, or mentally unstable; one is no longer rational or sane. Did you hear the crazy conspiracy theories the governor started peddling on the radio yesterday? I think that guy's cheese slid off his cracker! My aunt has always been a bit eccentric, but the cheese has really slid off the cracker in recent years. Andrew wants to sell bees as pets? Wow, his cheese has really slid off his cracker, huh?