The Collaborative International Dictionary of English v.0.48:

Rearmost \Rear"most`\ (-m[=o]st`), a.
   Farthest in the rear; last.
   [1913 Webster] Reremouse

WordNet (r) 3.0 (2006):

rearmost
    adj 1: located farthest to the rear [syn: backmost,
           hindermost, hindmost, rearmost]