One Donut Short of a Dozen 

Some one who is one donut short of a dozen is basically missing a key element to their brains. A.K.A., they are stupid.
One egg short of a dozen works too, but that sounds nasty.

P1: Look at Anna- ew, why is she hugging that cardboard cutout?
P2: Oh, she's one donut short of a dozen. She thinks that's her boyfriend.

One Donut Short of a Dozen by leigh_12 November 27, 2011