Dunderhead

On the same page as dubious is another powerful word, dunderhead.

Dunderhead comes from a Scottish dialect and means a stupid person. It is unequivocal and unambiguous, not dubious at all, perfectly clear. What's the word for that? Perspicuous?

Yes! I like it.

Perspicuous means having clarity of expression or style; lucid. From Latin.

So a dunderhead would rarely be perspicuous, though the meaning of the word is always crystal clear.