What's stolid? It sounds like someone who's stupid and built solid like a wall.

Well, you're right in the sense that stolid has its roots in the Latin adjective stolidus (which means dull, stupid, or foolish). Stolid (which has nothing to do a person's physical build) describes a person who is impassive — that is, someone who has or shows little or no emotion or sensibility.

So if you said, "Nick is as stolid as a wall" to mean that he's as expressive as a brick, then you'd be using the word correctly. (Of course, a Facebook wall is a different matter!) It won't mean, however, that he is physically strong or hard to knock over.

Thomas Hardy's The Mayor of Casterbridge shows you how to use the noun form of stolid:

Farfrae was always considerate to his fallen acquaintance; but it was impossible that he should not, by degrees, cease to regard the ex-corn-merchant as more than one of his other workmen. Henchard saw this, and concealed his feelings under a cover of stolidity . . .