noun, na-adjective
Refers to foolishness or a foolish act. Often used in formal or written contexts, and in compounds like 愚行 (foolish act) or 愚問 (foolish question).
彼の愚には呆れてものが言えない。
I'm speechless at his foolishness.
そんな愚を犯すとは思わなかった。
I didn't think you would commit such folly.
馬鹿 is a common, casual word for 'fool' or 'stupidity', while 愚 is more formal and often used in written language or set phrases.
愚か is an adjectival noun meaning 'foolish' and is more common in modern Japanese than the standalone 愚.
The kanji 愚 has been used historically to express foolishness or humility. The archaic pronoun usage derives from self-deprecating humble language.