When used as an adjective, it is the opposite of
clarity. It is a person, or a type of
communication (written, oral, digital, etc) that is
vague and unclear, despite volume.
It is prose that has a great deal of complicated information, but not simplicity.
If form follows function, people who are obtuse have little or no function. (Except perhaps to annoy the rest of us).
William F. Buckley comes to mind.