A general noun for a fool or an idiot; used in both spoken and written language, though it carries a somewhat strong derogatory tone.
そんなことをするとは、本当に愚か者だね。
Doing something like that makes you a real fool.
彼は一度失敗したが、すぐに立ち直り、愚か者とは言えない努力家になった。
He failed once but bounced back quickly and became a hard worker who cannot be called a fool.
昔話では愚か者が教訓を得る話がよく出てくる。
In folktales, stories about foolish people learning lessons often appear.