The Collaborative International Dictionary of English v.0.48:

Outermost \Out"er*most`\, a. [See Uttermost, Utmost, and cf.
   Outmost.]
   Being on the extreme external part; farthest outward; as, the
   outermost row. --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

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