rarefied vs unfrequent

rarefied

adj
  • Distant from the lives and everyday concerns of ordinary people; esoteric, exclusive, select. 

  • Elevated in style or nature, sublime; of high intellectual or moral value. 

  • Less dense than usual; thin. 

unfrequent

adj
  • Not frequent; not common; not happening often; infrequent. 

verb
  • To stop frequenting; cease to frequent. 

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