nonnegotiable vs sine qua non

nonnegotiable

noun

  • Something that is not negotiable. 

adj

  • Not negotiable; not subject to negotiation. 

sine qua non

noun

  • An essential or indispensable element, condition, or ingredient. 

How often have the words nonnegotiable and sine qua non occurred in a corpus of books? (source: Google Ngram Viewer )