you're slow in the head like a big pizza pie

Someone that is not very smart.
Someone that does something stupid.
Someone who is mentally challenged.

Billy: Horses don't have hooves until the farmer nails them on.
Jack: So, what do they have before that?
Billy: Feet?
Jack: ... You're slow in the head like a big pizza pie!

by Lalalala! May 17, 2006