In preparing future lessons on appositives, I came across the grammatical term oppositive, a new one for me.

Here's what one of the dictionary links (Dictionary.com, I think) offered:

oppositive

adj : expressing antithesis or opposition; "the adversative conjunction `but' in `poor but happy'" [syn: adversative]

So, but can function as an adversative or oppositive conjunction when followed by an antithetical modifier. That's pretty interesting to me. Hope it is to y'all.