Someone that can't integrate properly in society.
Someone that has a very low IQ usually due to learning disabilities of no fault of their own as opposed to a ning-nang or general idiot who is stupid because they never engaged their brain or tried to learn anything.
Used in a derogatory way to describe idiots who behave as if they never received an education or to decribe a person who behaves in a way that contradicts their IQ level.
The bus was full of nuff-nuffs going to special school.
The guy was a complete nuff-nuff.