google
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early 20th century (as a medical term denoting an adult with a mental age of about 8–12): from Greek mōron, neuter of mōros ‘foolish’.
wiktionary
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moron + -ic
etymonline
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moronic (adj.)
"of, pertaining to, or characteristic of morons," 1911, from moron + -ic. Related: Moronically.
