abstract idea vs whatsit

abstract idea

noun
  • An idea separated from a complex object, or from other ideas which naturally accompany it; as the solidity of marble when contemplated apart from its color or figure. 

  • Used other than figuratively or idiomatically: see abstract, idea. 

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 abstract idea and whatsit occurred in a corpus of books? (source: Google Ngram Viewer )