crass

The word crass entered English in the mid-16th century, directly from the Latin adjective crassus, which carried the primary meanings of "thick, solid, dense, or gross." This Latin term is of uncertain deeper origin, though it may be related to the Greek krateros (strong) or the Old Irish cress (fat). The semantic evolution from the physical property of thickness to the abstract quality of stupidity is a classic metaphorical shift. A "thick" or "dense" mind is one that is impervious to subtlety or refinement, leading to the modern sense of being crudely insensitive, obtuse, and lacking in discrimination.

The word is a simplex. It is a direct borrowing from Latin crassus, meaning "solid, thick, dense, fat."