aft vs postern

aft

adj
  • located at the back of a boat, ship, or airplane 

noun
  • The stern portion of a vessel. 

adv
  • At, near, or towards the stern of a vessel (with the frame of reference within the vessel). 

postern

adj
  • Situated at the rear; posterior. 

noun
  • A back gate, back door, side entrance, or other gateway distinct from the main entrance. 

  • A subterranean passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks. 

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