oppugn vs skirmish

oppugn

verb
  • To contradict or controvert; to oppose; to challenge or question the truth or validity of a given statement. 

skirmish

verb
  • To engage in a minor battle or dispute 

noun
  • A brief battle between small groups, usually part of a longer or larger battle or war. 

  • Any minor dispute. 

  • A type of outdoor military style game using paintball or similar weapons. 

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