When you've got some chick (or dude, depending on your preference) sucking your dick, then you tell them to hold on because you have to pee/left the oven on/phone is ringing/whatever, then you stick your spit-slathered dong in the vacuum bag and come back and cockslap them as hard as you possibly can. Originally named for Wilford Brimley, but "Dusty Wilson" had a better ring to it.
-Dude, my girl just broke up with me after I gave her a dusty wilson.
-Well what do you expect? That shit's gross.