There are other cognates derived from the same Latin root ἴδι‑ (ídi‑) of the adjective ἴδιος (ídios). Let us look at two of the most common ones, namely Sp. idiota ~ Eng. idiot and Eng. idiosyncrasy ~ Sp. idiosincrasia. The cognates Sp. idiota ~ Eng. idiot are learned words that are attested as early as the 13th century in both languages. We know that English got this word from French and Spanish probably did too since Fr. idiote is already attested in the preceding century. The ultimate source of these loanwords is Greek ἰδιώτης (idiṓtēs) ‘layman, person not involved in public affairs’. Latin borrowed this word as ĭdĭōta with the same meaning of ‘ordinary person, layman’ but eventually by Late Latin times it came to have the meaning ‘uneducated or ignorant person’. The Greek word ἰδιώτης (idiṓtēs, idi‑ṓt‑ēs) was derived from the adjective ἴδιος (ídios) ‘one’s own, private’ and the suffix ‑ώτ‑ης (‑ṓt‑ēs) that derived agent nouns (feminine ‑ώτ‑α, ‑ṓt‑a).
Both Sp. idiota and Eng. idiot were originally used with the original Greek meaning ‘non-expert, layman, ignorant (not knowing)’, though that meaning is now obsolete for both words. The current meaning of these words go from ‘lacking in intelligence or good sense’ to something stronger, a meaning that is already attested in English by the 15th century, in French in the mid-17th century. In Spanish, however, that sense did not appear in the DRAE for Sp. idiota until the 1869 edition of the dictionary.
In legal and psychiatric technical terminology, the word idiot has been used in the past with the meaning ‘a person so profoundly disabled in mental function or intellect as to be incapable of ordinary acts of reasoning or rational conduct; specially a person permanently so affected, as distinguished from one with a temporary severe mental illness’ (OED). This ‘mental illness’ sense of the word was common in the 19th and 20th centuries, but its roots go back a few centuries. The sense came to be found in this word’s French and Spanish cognates as well. It is not in current use, however, and thus, it is a historical sense. The mental illness referred to in this sense went by the name of idiocy (cf. Sp. idiocia, which was rare, or idiotez; Fr. idiotie). The AHD describes this sense of idiot as ‘a person of profound mental retardation having a mental age below three years and generally being unable to learn connected speech or guard against common dangers’ and warns us that ‘the term belongs to a classification system no longer in use and is now considered offensive’. Other technical terms that were likewise used in the past but are not in use today and are considered offensive are moron (Sp. retrasado/a mental), imbecile (Sp. imbécil), and cretin (Sp. cretino).
Eng. idiot and Sp. idiota are nouns, as were the words that they come from. There was an adjective derived from this noun in the source words, namely Late Lat. ĭdĭōtĭcus, and Ancient Greek ἰδιωτικός (idiotikós) ‘private’. In Late Latin, this adjective came to mean ‘uneducated, ignorant, unskillful’ (L&S). English borrowed it in the early 18th century as idiotic, as in Stop asking idiotic questions, What she did was idiotic, or Her behavior was totally idiotic. This word’s synonym idiotical is already attested in the 1640s, almost a century earlier. Spanish did not borrow this adjective and so the English adjective idiotic is translated by using the noun idiota as an adjective, which is not very common, or by means of other related or synonymous words, as in Deja de hacer preguntas imbéciles ‘Stop asking idiotic questions’ (using the noun/adjective imbécil instead), and Lo que hizo fue una idiotez (or una imbecilidad) ‘What she did was idiotic’ (using the nouns idiotez ‘idiotic thing, etc.’ or imbecilidad 'stupid thing', a noun derived from the noun/adjective imbécil 'stupid, imbecile').
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 Henry H. Goddard, an American psychologist and eugenicist, proposed in the early 20th century a classification system for intellectual disability that was based on the Binet-Simon concept of mental age. Those with a mental age of less than three years were identified as idiots; those with a mental age of three to seven years were classified as imbeciles; and those with a mental age of seven to ten years were called morons.