idiotism

From idiot +‎ -ism.

idiotism (countable and uncountable, plural idiotisms)

1. (now chiefly historical) Very severe mental retardation.
   • 1997, Roy Porter, The Greatest Benefit to Mankind, Folio Society 2016, p. 488:

     Idiotism had long been accepted as hopeless: ‘Absolute idiocy admits of no cure,’ noted the nineteenth-century psychiatrist George Man Burrows (1771–1846).

2. A foolish utterance.
   • 1863, Sheridan Le Fanu, The House by the Churchyard

     […] that clear soprano, in nursery, rings out a shower of innocent idiotisms over the half-stripped baby, and suspends the bawl upon its lips.

From Latin idiotismus.

idiotism (plural idiotisms)

1. Idiom.
2. An overly literal translation of an idiom.