WordNet sense 1 (relatively dense in consistency):

thick

WordNet sense 2 (not admitting of penetration or passage into or through):

impenetrable

WordNet sense 3 (of comparatively great physical weight or density):

heavy

WordNet sense 4 (having component parts closely crowded together):

compact, compact

WordNet sense 5 (gathered together or made less diffuse):

concentrated

WordNet sense 6 (slow to learn or understand; lacking intellectual acuity):

dim, dull, dumb, obtuse, slow

WordNet sense 7 (lacking or marked by lack of intellectual acuity):

stupid