The Collaborative International Dictionary of English v.0.48:

Inmost \In"most`\, a. [OE. innemest, AS. innemest, a double
   superlative form fr. inne within, fr. in in. The modern form
   is due to confusion with most. See In, and cf. Aftermost,
   Foremost, Innermost.]
   Deepest within; farthest from the surface or external part;
   innermost.
   [1913 Webster]

         And pierce the inmost center of the earth. --Shak.
   [1913 Webster]

         The silent, slow, consuming fires,
         Which on my inmost vitals prey.          --Addison.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "inmost":
   anonymous, central, closet, deep, incognito, indoor, inner,
   innermost, inside, interior, internal, intestine, intimate,
   intrinsic, inward, isolated, personal, private, privy, retired,
   secluded, sequestered, withdrawn