Derives from how Wendy's (and other fast food places) sometimes have open air ice machines, letting dust, and other foreign objects fall in.
This should not be ingested or even put in drinks.
Unless you're fond of Herpes and Gonorrhea.
Dave: "Dude, what's wrong?"
Clayton: "I think I have Herpes. I totally had Wendy's ice yesterday."
Dave: "Bad move, man."