It would seem that 'dense' would be 'packed with brains', which is the opposite of stupid.
Dense comes from the Latin
densus. Densus means
close together with the predominant idea of impenetrability. It is the opposite of
What I get from that is:
- An intelligent mind is nimble and quick. A "stupid" mind is not nimble, and takes a longer time to pick up an idea, to absorb a new idea.
- If you consider an idea as an outside light, an intelligent mind can "receive illumination" because it is not opaque, not
dense. An intelligent mind is transparent enough to receive new information and act on it.
Dense implies a longer time for new information to penetrate the mind and be absorbed as information worth acting on.
As others have noted, dense in this sense is a synonym for thick.
The OED records the first usage of dense in this sense in 1822:
1822 C. Lamb in London Mag. Apr. 307/2 "More virtuous than myself, or more dense".
But "thick" was used much earlier, with the first citation in Shakespeare:
1600 Shakespeare Henry IV, Pt. 2 ii. iv. 243 "Hang him baboon, his wit's as thicke as Tewksbury mustard".
Note the reference to thick mustard, which I take as alluding to its 'difficulty to stir'.