Open English Wordnet
Adjectives
(s)dense, heavy, impenetrablepermitting little if any light to pass through because of denseness of matter“dense smoke”, “heavy fog”, “impenetrable gloom”
(s)dense, dim, dull, dumb, obtuse, slowslow to learn or understand; lacking intellectual acuity“so dense he never understands anything I say to him”, “never met anyone quite so dim”, “although dull at classical learning, at mathematics he was uncommonly quick” (Thackeray), “dumb officials make some really dumb decisions”, “he was either normally stupid or being deliberately obtuse”, “worked with the slow students”