A thing (used in a vague way to refer to something whose name one cannot recall, or that one is embarrassed to say)
Pass me one of those whatsits, will you?.
You use whatsit instead of a noun or name which you cannot remember or which you do not want to say because it is rude. We wanted to be here early in case the whatsit, maintenance supervisor had forgotten to deal with it. a word you use when you cannot think of the word you want (what's it?)