The Collaborative International Dictionary of English v.0.48:

Outmost \Out"most`\, a. [OE. outemest, utmest, AS. [=u]temest, a
   superl. fr. [=u]te out. See Out, Utmost, and cf.
   Outermost.]
   Farthest from the middle or interior; farthest outward;
   outermost.
   [1913 Webster]

WordNet (r) 3.0 (2006):

outmost
    adj 1: situated at the farthest possible point from a center
           [syn: outermost, outmost]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "outmost":
   apparent, cortical, epidermic, exomorphic, exterior, external,
   extrinsic, farthest, fringe, furthermost, furthest, open, out,
   outer, outermost, outlying, outside, outstanding, outward,
   outward-facing, peripheral, public, remotest, roundabout, seeming,
   superficial, surface, utmost, uttermost