mourn vs nightmare

mourn

verb
  • To utter in a sorrowful manner. 

  • To express sadness or sorrow for; to grieve over (especially a death). 

  • To wear mourning. 

noun
  • Sorrow, grief. 

  • A ring fitted upon the head of a lance to prevent wounding an adversary in tilting. 

nightmare

verb
  • To have a nightmare. 

noun
  • Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure. 

  • A very bad or frightening dream. 

  • A feeling of extreme anxiety or suffocation experienced during sleep; Sleep paralysis. 

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