village idiot 
noun chiefly humorous
A person of very low intelligence resident in a village.I remember it had a village green with a village idiot sitting there....
- He turns out to be the clown (almost the village idiot, frankly) of the evening.
- At first, they pick on a village idiot, with the mental age of a child, and when that doesn't work, pick on someone else and then someone else.