duress vs helotism

duress

noun
  • Restraint in which a person is influenced, whether by lawful or unlawful forceful compulsion of their liberty by monition or implementation of physical enforcement; legally for the incurring of civil liability, of a citizen's arrest, or of subrogation, or illegally for the committing of an offense, of forcing a contract, or of using threats. 

  • Constraint by threat. 

verb
  • To put under duress; to pressure. 

helotism

noun
  • The permanent oppression of a nominally free group of people 

  • A form of mutualism in which one species is forced to perform tasks for another, for their mutual benefit 

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