덜떨어지다 is a verb, which means "to be weak in the head.", "to be short on brains." Example: 그는 건장하고 다부진 남자였지만 가끔 덜떨어진 행동을 하는 경향이 있다. He was a big strapping guy but he sometimes acts like a fool.

I think its mean is "dumbass". Source: endic.naver.com