Emily Dickinson Lexicon

ID'I-OT-ISM, n. [Fr. idiotisme; It. and Sp. idiotismo; Gr. ιδιωτισμος, a form of speech taken from the vulgar, from ιδιος.]

1. An idiom; a peculiarity of expression; a mode of expression peculiar to a language; a peculiarity in the structure of words and phrases. Scholars sometimes give terminations and idiotisms, suitable to their native language, to words newly invented. Hale.
2. Idiocy. Beddoes, Hygeia. But it would be well to restrain this word to its proper signification, and keep idiocy and idiotism distinct.