덜렁거리다 (or 덜렁대다) has two major meanings.
1. a loosely connected object dangles around or makes such a noise. e.g. 가방 끈이 끊어져 덜렁거렸다.
2. A person is disorganized and behaves erratically lacking composure (syn: 경망스럽다, 가볍다; ant: 침착하다, 차분하다, 야무지다). e.g. 얘는 늘 침착하지 못하고 덜렁거린다.
It is an example of a verb describing a sound or motion. There's a group of them ending in -거리다, -대다, or -하다.
- 덜컹거리다/대다 = Something hefty rattles (as a car would on a rough surface).
- 퍼덕거리다/퍼덕대다/퍼덕이다 = makes a flapping motion/noise, like a bird's wings, fish out of water, a flag in the wind, etc.
- 펄럭거리다/펄럭대다/펄럭이다 = similar but in a lighter and free movement, so it suits a flag or cloth in the wind.