It is an abstract term, where the observer assigns beauty to an object because it is in an unfinished state. It's poetic in nature, and implies that something is beautiful because it is unfinished...which is unusual because things are usually considered beautiful when completed and whole. However, this is all personal viewpoint. For example, a piece of music might be considered beautifully unfinished by some...because it lacks an ending. In short, it is a very term of phrase that you won't see very often at all.

There is finishing on furniture sometimes that isn't a good fit or something that you started but is incomplete. So "beautifully unfinished" just means that it is beautiful without the finish or being complete depending on context.