extraordinary vs unheard-of

extraordinary

adj

  • Not ordinary; exceptional; unusual. 

  • Remarkably good. 

  • Special or supernumerary. 

noun

  • Anything that goes beyond what is ordinary. 

unheard-of

adj

  • Previously unknown; unprecedented. 

  • Unknown or known to only few people. 

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