One who doesn't understand the world around them.
One who is unable to understand, to speak or to form creative ideas or thoughts.
'Greg was unable to form a proper sentance.'
'Greg is a moron.'
'Greg forgot how to read, so his homework was incomplete, therefore Greg is a moron.'
'Greg can't understand when he has lost, because he is a moron!"
The only way to deal with this is to treat everybody as a moron till proven otherwise.
How? Explain things to them like you would a child and hope they dont make a mistake