Used negatively to criticize someone's intelligence or decisions. Calling someone a dumbo after they make a mistake is mean.
Pro Tip 2/3
Informal Speech
Mostly used in casual conversations among friends, not formal situations. I joked with my friend, 'You locked your keys in the car again? What a dumbo!'
Pro Tip 3/3
Emotional Impact
Can hurt feelings if used seriously; best reserved for lighthearted teasing. He felt bad after his brother called him a dumbo for dropping the vase.