1910, medical Latin, "one of the highest class of feeble-minded persons," from Greek (Attic) mōron, neuter of mōros "foolish, dull, sluggish, stupid," a word of uncertain origin. The former connection with Sanskrit murah "idiotic" (see moratorium) is in doubt. 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]