ABSURDITY, n. A statement or belief manifestly inconsistent with one’s own opinion.

Ambrose Bierce (1842-1914?) American writer and journalist

“Absurdity,” The Cynic’s Word Book (1906)
    (Source)

Included in The Devil's Dictionary (1911).

In later versions, Bierce added:

2. The argument of an opponent. A belief in which one has not had the misfortune to be instructed.