껄렁 has a very negative connotation. It describes a person or their behavior which is unreliable, ungainly, and frivolous, i.e, bad comportment. 걸음걸이는 껄렁(하다) => His gait is ungainly. 껄렁대는 사람 => a person behaving in impolite and untoward manners. Both 껄렁대다 and 껄렁거리다 mean acting in such a way, and 껄렇하다 is an adjective for it.