unfrequent vs unthinkable

unfrequent

adj
  • Not frequent; not common; not happening often; infrequent. 

verb
  • To stop frequenting; cease to frequent. 

unthinkable

adj
  • Inconceivable or unimaginable; extremely improbable in a way that goes against common sense. 

  • Incapable of being believed; incredible. 

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