This is the sort of question that pops into my head when I'm feeling frayed around the edges. It turns out that "to whelm" means to turn something, such as a dish, upside down in order to cover it. So I guess the question of whether something is "overwhelming" or "underwhelming" turns on the amount of coverage a particular whelming provides. As the Valley Girls might have said in an alternate universe, "Well, whelm me with a Tupperware. . . and be sure to pick the right number."