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1.
An item that you cannot think of at the given moment. A Thingamajig. See Also widget.
Hand me that whosawhatsit over there. Not that one. The one next to the thingy with that thing on it.
by Jacob McClenahan June 21, 2005
 
2.
What you say when someone speaks at you and you didn't hear what they said.
Person A: Do you want to have dinner with Jack and Jill tonight?
Person B: {looks up from book they were reading} Whosawhatsit?
by markds75 September 05, 2006