Happy Bat
You're totally bats! He's batty! They've got bats in the belfry! That idea is bat, um, poop! These winged mammals sure do know how to carry off a good insult. Why go through all the linguistic gyrations to explain that someone isn't the sharpest tool in the shed (What shed? What tool? I mean, hammers aren't even supposed to be sharp!)...when you could just compare them to a friendly chiroptera! So the next time you really want to let someone know you think they're unglued, don't reach for some kind of "nuts-based" metaphor or "loose screws" verbal device. Reach for this bat instead!