emeritus vs prehistoric

emeritus

adj
  • Retired, but retaining an honorific version of a previous title. 

noun
  • A person retired in this sense. 

prehistoric

adj
  • Ancient; very old, outdated, etc. 

  • Of or relating to the epoch before written record. 

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