astern vs rearmost

astern

adj
  • Behind a vessel; having a bearing of 180 degrees from ahead. 

adv
  • Behind (a vessel); in the rear. 

  • In the direction of the stern; backward (motion); to the rear. 

rearmost

adj
  • Furthest to the rear. 

  • The rearmost member of the strike team walked backward to guard them from a surprise attack from behind. 

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