I've no idea, even the internets aren't sure. My east coast friend says the term goes back to the 80s if not earlier. White Castle made these little nasty addictive burgers that were cooked on a grill without flipping, just put the onions and meat on the grill, top with the bun to seal in the damp grease, and slide them along until they are done. The world's saddest assembly line. They sold "sliders" 5 for a dollar or something like that.