It would seem that 'dense' would be 'packed with brains', which is the opposite of stupid.

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    I always associated it with being thick-skulled, i.e. you've got lots of dense material to break through.
    – thomj1332
    Commented Aug 10, 2017 at 22:40
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    Well, packed with something, anyway. Note that if something is dense, it is harder to move or change. In the usual Conduit Metaphor, one speaks of 'getting an idea through' to someone. If their head is full of rocks or something equally dense, the idea doesn't get through and they're impermeable. Commented Aug 10, 2017 at 22:43
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    Don't know the order, but thick is used similarly
    – Unrelated
    Commented Aug 11, 2017 at 0:09
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    Can we be sure it originally referred to brains? Wasn't the brain being the seat of intelligence still something of an arguable point when the idiom first appeared?
    – Phil Sweet
    Commented Aug 11, 2017 at 0:37

2 Answers 2


Dense comes from the Latin densus. Densus means thick, close together with the predominant idea of impenetrability. It is the opposite of fluid

What I get from that is:

  1. 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.
  2. 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'.

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