notion vs whatsit

notion

noun
  • Mental apprehension of whatever may be known, thought, or imagined; idea, concept. 

  • A sentiment; an opinion. 

  • An invention; an ingenious device; a knickknack. 

  • Any small article used in sewing and haberdashery, either for attachment to garments or as a tool, such as a button, zipper, or thimble. 

  • Inclination; intention; disposition. 

whatsit

noun
  • A thing (used in a vague way to refer to something whose name one cannot recall, or that one is embarrassed to say) 

How often have the words notion and whatsit occurred in a corpus of books? (source: Google Ngram Viewer )