A derogatory term used by mentally challenged or very low IQ institutionalized males or females used to put down another mentally challenged person. Sometimes used against their careworkers. Normally used in a retallitory manner and refering to the others IQ. I personally never experienced this term outside of an institutioanlized situation. And found this was this worst verbal insult said person could use. If you are called a low grade you have not been complimented. I like to used the term in the car to prevent road rage as it is safe around children and it really helps as only I (until now) know how bad I've insulted the driver of the other vehicle.
Oh you think I'm a dummy. Well you're a low grade. Low grade, low grade, low grade!
Children you should never drive like that. It is unsafe and that person is a low grade.