My own creation, for something you can't define, or don't know / can't remember the name of at the moment, such as a part of something you're trying to assemble or a piece of your car that has come off in your hand. Interchangeable with words like thingy, dohickey, and the like, but for some reason doesn't get the looks of contempt from mechanics and the like that those words do ;-)
Friend 1: "What are we supposed to do next?"
Friend 2: "I think we need to put one end of that whoopdie over there into this clamp."
Mechanic: "What seems to be the problem with your car?"
Me: "When I went to open the hood, that little whoopdie came off in my hand."