The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster] 2. (Naut.) Nearest the stern; most aft. [1913 Webster]WordNet (r) 3.0 (2006):
aftermost adj 1: located closest to the stern or tail