Several bricks short of a load (pallet). Several sandwiches short of a picnic. Several sheets short of a ream. There are quite a few real nasty ones, and not appropriate for this forum. A 'stupid' person is just another person who has (had) an unfortunate development.
Everyone makes an occasional mistake Rev. but if you "treat people as if they were what they ought to be, you can help them to become what they are capable of being (Geothe)" and you'll have a much happier life yourself.