una persona que no es inteligente
Dumbbell (this is mean)
The cage is turning but the hamster died (humorous)
The lights are on but nobody is home (also humorous)
Ignorant, stupid, retarded... estas son las mismas como...ignorante, estúpido, y retrasar...
You could simply say, Uneducated. But if you want to put him down say: He is as thick as six short planks. That's quite a common phrase in England.