impenetrable vs inappropo

impenetrable

adj

  • Not penetrable. 

  • Opaque; obscure; not translucent or transparent. 

  • Incomprehensible; fathomless; inscrutable. 

noun

  • A person not openly given to friendship. 

inappropo

adj

  • Not apropos. 

  • Inappropriate. 

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