Describes a person, action, or idea as lacking good judgment or intelligence. Can be used critically or self-deprecatingly.
Don't say such a foolish thing.
He foolishly turned down the offer.
He is truly a foolish person.
Don't do something so foolish.
It was foolish to trust him.
愚か is the standard modern adjective for describing something foolish, whereas 烏滸 is an archaic noun/adjectival noun with a similar meaning.
愚か is a stronger, more direct word for foolishness, whereas 鈍い softens the criticism to mean merely slow to understand.
愚か describes foolish behavior or poor judgment, while 無知 specifically points to a lack of information or awareness that may or may not lead to foolish acts.
愚か means foolish or unwise, the direct opposite of 賢明's sense of sound judgment.