Source : Webster's Revised Unabridged Dictionary (1913)

Aftermost \Aft"er*most\, a. superl. [OE. eftemest, AS.
   [ae]ftemest,akin to Gothic aftumist and aftuma, the last,
   orig. a superlative of of, with the superlative endings -te,
   -me, -st.]
   1. Hindmost; -- opposed to {foremost}.

   2. (Naut.) Nearest the stern; most aft.

Source : WordNet®

aftermost
     adj : located closest to the stern or tail