Dusty hunting is when you comb liquor stores looking for old bottles of whiskey. The reason dusty hunting makes sense is because whiskey doesn't spoil, so many liquor stores don't worry about rotating old stock. Believe it or not, you can find bottles that have been sitting on the shelf for 30 years, hence "dusty."
The point of dusty hunting is that you can find really great stuff, from distilleries that are no longer producing, usually pretty cheap since just like they don't rotate old stock, they also often don't reprice it either.
If you want to know more, check out Greg Gilbert's blog, Bourbon Dork. He has posted a three-part series on the fine art of dusty hunting. Part one is here, part two is here, and part three is here. While you're there, check out some of his other posts. He has a lot of good stuff. He explains the weirdness that is Four Roses pretty well here.