dress that is not casual, but not necessarily formal

Over this side of the Pond we just talk about "smart casual" wear, Bill. Suits me .

I think uncasual has a place as an ironic term, though.

Think about the difference between:

1. He had a formal manner.

2. He had a casual manner.

3. He had an uncasual manner.

To me it implies that the subject is trying to look casual (and relaxed) but is failing to carry it off. Stilted, perhaps.