merciful vs rigid

merciful

adj
  • Showing mercy. 

rigid

noun
  • An airship whose shape is maintained solely by an internal and/or external rigid structural framework, without using internal gas pressure to stiffen the vehicle (the lifting gas is at atmospheric pressure); typically also equipped with multiple redundant gasbags, unlike other types of airship. 

  • A bicycle with no suspension system. 

adj
  • Stiff, rather than flexible. 

  • Fixed, rather than moving. 

  • Uncompromising. 

  • Rigorous and unbending. 

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