That thing you are thinking of that you just cant remember the word for at that time. When saying it, people often attempt to explain it with accompanying hand/arm/body movements.
Person 1: That...uh...thing! The thingy thing! You know the one i mean!
Person 2: Oh! That thing! Its over there!
a something, a thingymajig,a watchamacalit,a doo dad, a habba whatsa, a do hickey. The word that replaces a noun that you cant think of at that certain moment.