cassock vs nightgown

cassock

noun
  • An item of clerical clothing: a long, sheath-like, close-fitting, ankle-length robe worn by clergy members of some Christian denominations. 

nightgown

noun
  • A long loose robe worn mainly by women for sleeping in. 

  • A dressing gown. 

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