dense

The word evokes a sense of 'tightness' or 'lack of space,' whether that is physical matter, vegetation, or mental processing. It describes things that are hard to penetrate—be it a wall of trees, a heavy metal, or a difficult concept. In a physical context, it is neutral and scientific. When describing forests or crowds, it implies an obstacle or a feeling of being enclosed. When applied to people, the term becomes pejorative and informal. Unlike 'stupid,' which can be a general trait, calling someone 'dense' often suggests they are failing to grasp something that should be obvious, as if their mind is too "thick" for the information to penetrate.