Literary and somewhat formal. Often used in self-deprecating polite speech (我愚钝 = 'I'm slow / forgive my dullness'). For an everyday insult use 笨 (bèn) or 傻 (shǎ); 愚钝 is more bookish.
memory
愚 (foolish) + 钝 (blunt, dull — same character used for blunt knives). Mind blunt as a butter knife.
