The clear implication of belming at someone is that either;
- You don't really believe a word they are saying.
- You think they're a MONG and not really worthy of proper conduct.
The act of belming can be enhanced in either of two ways, preferably both;
- Stroking the extended chin at the same time. The longer the strokes, the more you're infering that lies are being told.
- Saying the world 'belm' at the same time. Quite a difficult word to say clearly when your tongue is sticking your lower lip out, but effective all the same.