When you call something dusty, it's either literally covered in dust, or so old and unoriginal that it might as well be.
You can use a feather duster to clean off dusty surfaces, but if something is a dusty color, that just means it's got a grayish tint to it, like dusty pink or dusty blue. If something has been around for a really long time and it's worn-out, stale, or unoriginal, you can also call it dusty. If you're a stand up comedian whose been working the same act for thirty years, chances are your jokes are getting a little dusty.
