A mentally retarded person whose intellectual development proceeds normally up to about the eighth year of age and is then arrested so that there is little or no further development; an adult having the mental development of an 8-to-12-year old. A moron is considered capable of doing routine work under supervision.
Hence, a very stupid person.
(mo·ron) NOUN: According to psychologists of the past, a moron was a person with a cognitive disability having a mental age of from 7 to 12 years according to the Binet intelligence tests. Such a person generally had communication and social skills enabling some degree of academic or vocational education. The term, coined by H.H. Goddard in 1910, belongs to a classification system no longer in use and is now considered offensive.