From Longman Dictionary of Contemporary Englishwhatsitwhats‧it /ˈwɒtsɪt $ ˈwɑːts-, ˈwʌts-/ noun [countable] spokenTHINGNAME OF A THING a word you use when you cannot think of the word you want Try and undo the screw to get the whatsit off.
Examples from the Corpus
whatsit• Sick of Neighbours It's my neighbour Deardrie - he's a right old interfering whatsit.• The curtains were green with little whatsits along the edge.• Something decidedly peculiar about the boy with the whatsit round his head.• Just enough to work up a sweat, keep the blood moving, tone the old cardio-vascular whatsit.Origin whatsit (1800-1900) what's it? 