corvee vs forced labor

corvee

noun
  • Unpaid labor required by a feudal lord. 

  • Labor, especially for roads or dams, in lieu of taxes. 

forced labor

noun
  • Work which one is compelled to perform against one's will, especially in a condition of involuntary servitude as a prisoner or slave. 

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