Hi, An 어리석은 person is someone who is not wise. They make mistakes, because they don't know how to make wise decisions. Also, an 어리석은 thing is a mistake - something you regret having done. On the other hand, a 미련한 person is someone who's dumb, lazy, idle, and silly. In Korea, when someone's really 미련하다 we say they're "as 미련하다 as a bear," because bears are usually considered slow, clumsy and idle (In reality, they're not. But that's beside the point). I'd say 미련하다 is a little bit more derogatory and hurtful than 어리석다. 어리석다 usually refers to a stupid thing someone did, while 미련하다 is more of a direct criticism of one's character. But that's just my impression. Bottom line: 어리석다 = the opposite of wise. 미련하다 = the opposite of sharp, smart, quick.