he was fallen from the roof but he couldn't reach to the ground cuz he was caught on the brench of the tree. so u can say it's not completely fallen. it's a play on words. 덜 means deficient,not complete 떨어지다 means fall and 덜떨어지다 has own meaning by itself. it means there is some dirt on his head like a children. means still thinking or doing like a child and not completely grown yet. they use this word(덜떨어지다(덜떨어진:adjective)) for weird person or slow or childish or someone who did a ridiculous thing or dummy or.... e.g. 덜떨어진 녀석. 덜떨어진 여자. 덜떨어진 남자. 덜떨어진 사람...

I fell off the top of a building. But I was hooked on a tree's branch. So I was safe. Then what do you call this situation? The answer is ... The person who fall off insufficient (in korea it means retard) Say, 덜 떨어진 사람 = retard in Korea