If an activity, event or deal is done and dusted, it has been completed successfully and none of remains to be done.
I’ve read many suggestions as to the source, but this one is the most sensible, or, at least, the most interesting. In the past, before blotting paper was created, documents were signed using pens dipped in ink. Drying time could be considerable because the application of the ink varied so much. To speed things up, an absorbent dust was sprinkled on the fresh writing, left momentarily, and then tipped off the sheet. Now, with the ink dry and unlikely to smear, the document could be rolled or folded, and carried away. Thus, a deal was considered finalized when it was “done and dusted.”
An alternative, with exactly the same meaning, is “home and dry” (British) or “home free” (American).
These two idioms mean pretty much the same thing as “safe and sound,” but specifically safe at home. “Dry” refers to being safe from the elements, such as stormy weather, and thus is particularly relevant to the Wet Coast, the rainforest of western North America. Some sources trace these phrases back to the late 19th century.
“Home and dry” reminds me, however, of the expression “rode hard and put away wet,” originating from the south and west United States. The phrase itself comes from horseback riding. When a horse runs, it works up a sweat. Horses that are put away wet with sweat may suffer from muscle stiffness and chills. Before being put back in the stable, it should be allowed to cool down by walking, then rubbed down and its body covered with a blanket.
Which means the horse is home and dry. And my column for today is done and dusted!