Idiocy is a state of mental unsoundness amounting almost or quite to total absence of understanding. Imbecility is a condition of mental weakness, which may or may not be as complete as that of idiocy, but is at least such as to incapacitate for the serious duties of life.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

stupidity There's a fine line between bravery and stupidity.
folly Who lives without folly is not so wise as he thinks.
foolishness I'm getting endlessly annoyed by this foolishness.
innocence At first everybody was convinced of his innocence.
insanity Love is a temporary insanity curable by marriage.

Popularity over time

Word	1850 → 2019	Trend
idiocy		steady
stupidity		rising
folly		steady
foolishness		rising
imbecility		falling
innocence		rising

Use in printed books, per decade from 1850 to 2019.

By length

5 letters: folly²
7 letters: fatuity, amentia, idiotcy
8 letters: insanity⁴, idiotism, idiocity, idioticy, idiotacy, idiotery
9 letters: innocence³, stupidity⁴, silliness³, idiotness
10 letters: imbecility, moronicity, idioticity
11 letters: foolishness³, idioticness
13 letters: senselessness, idioticalness

The small number after each word is its number of syllables (folly² = 2 syllables).

In other languages

Français: idiotie
Italiano: idiozia
Español: idiotez
Deutsch: Idiotie
Català: idiotesa