na-adjective, noun
foolish; imbecilic; feebleminded
A formal, somewhat literary term for a lack of intelligence or wisdom, often used in historical or critical contexts. Not a casual insult.
その暗愚な政策は国を危機に陥れた。
That foolish policy plunged the country into crisis.
彼は暗愚の君主として歴史に名を残した。
He went down in history as a feebleminded monarch.
Compound of 暗 (dark, unclear) and 愚 (foolish). The combination suggests a mind clouded by ignorance.