Wiktionary
whaddayacallit
n. (non-gloss definition: A metasyntactic term used for any object whose actual name the speaker does not know or cannot remember.)
Usage examples of "whaddayacallit".
But I saw you doing your crossword puzzle, and I let my whaddayacallit, my joie de vivre, get the better of me, and the next thing I know, I'm spending a whole lot more money than I should on your new book.