Dense

From Middle French dense, from Latin densus.

Full definition of dense

1918, W. B. Maxwell, The Mirror and the Lamp Chapter 13, And Vickers launched forth into a tirade very different from his platform utterances. He spoke with extreme contempt of the dense stupidity exhibited on all occasions by the working classes. He said that if you wanted to do anything for them, you must rule them, not pamper them.

Opaque; allowing little light to pass through.
Obscure, or difficult to understand.
(mathematics, topology) Being a subset of a topological space that approximates the space well. See Wikipedia article on dense sets for mathematical definition.
Of a person, slow to comprehend; of low intelligence.

(having relatively high density) solid
(crowded together) compact, crowded, packed
(difficult to penetrate) thick, solid
(allowing little light to pass through) cloudy, opaque
(difficult to understand) abstruse, difficult, hard, incomprehensible, obscure, tough
(slow to comprehend) dumb, slow, stupid, thick

(having relatively high density)
(crowded together) diffuse, few and far between of things as opposed to one thing, scattered, sparse, rarefied scientific, to describe gases
(difficult to penetrate) thin
(allowing little light to pass through) clear, diaphanous, see-through, translucent, transparent
(difficult to understand) clear, comprehensible, easy, simple, straightforward, understandable
(in mathematics) meager
(slow to comprehend) bright, canny, intelligent, quick, quick-witted, smart