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1910, medical Latin, from Greek (Attic) moron, neuter of moros "foolish, dull, sluggish, stupid," probably cognate with Sanskrit murah "idiotic." Latin morus "foolish" is a loan-word from Greek. Adopted by the American Association for the Study of the Feeble-minded with a technical definition "adult with a mental age between 8 and 12;" used as an insult since 1922 and subsequently dropped from technical use. Linnæus had introduced morisis "idiocy."
The feeble-minded may be divided into: (1) Those who are totally arrested before the age of three so that they show the attainment of a two-year-old child or less; these are the idiots. (2) Those so retarded that they become permanently arrested between the ages of three and seven; these are imbeciles. (3) Those so retarded that they become arrested between the ages of seven and twelve; these were formerly called feeble-minded, the same term that is applied to the whole group. We are now proposing to call them morons, this word being the Greek for "fool." The English word "fool" as formerly used describes exactly this grade of child--one who is deficient in judgment or sense. [Henry H. Goddard, in Journal of Proceedings and Addresses" of the National Education Association of the United States, July 1910]
moron mo·ron (môr'ŏn')
A person of mild mental retardation having a mental age of from 7 to 12 years and generally having communication and social skills enabling some degree of academic or vocational education. The term belongs to a classification system no longer in use and is now considered offensive.