Webster's Revised Unabridged Dictionary · 1913

Idiotism

Idiotism , noun

[French idiotisme, Latin idiotismus the way of fashion of a private person, the common or vulgar manner of speaking, Greek {not transcribed}, from {not transcribed} to put into or use common language, from {not transcribed}. See Idiot.]

1.
An idiom; a form, mode of expression, or signification, peculiar to a language.
Scholars sometimes give terminations and idiotisms, suitable to their native language, unto words newly invented. — M. Hale
2.
Lack of knowledge or mental capacity; idiocy; foolishness.
Worse than mere ignorance or idiotism. — Shaftesbury
The running that adventure is the greatist idiotism. — Hammond