The Collaborative International Dictionary of English v.0.48:

Adumbrate \Ad*um"brate\, v. t. [L. adumbratus, p. p. of
   adumbrare; ad + umbrare to shade; umbra shadow.]
   [1913 Webster]
   1. To give a faint shadow or slight representation of; to
      outline; to shadow forth.
      [1913 Webster]

            Both in the vastness and the richness of the visible
            universe the invisible God is adumbrated. --L.
                                                  Taylor.
      [1913 Webster]

   2. To overshadow; to shade.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adumbrate
    v 1: describe roughly or briefly or give the main points or
         summary of; "sketch the outline of the book"; "outline his
         ideas" [syn: sketch, outline, adumbrate]
    2: give to understand; "I insinuated that I did not like his
       wife" [syn: intimate, adumbrate, insinuate]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "adumbrate":
   allude to, argue, augur, auspicate, becloud, bedim, bespeak,
   betoken, block out, bode, body forth, call, chalk out,
   characterize, cloud, darken, demonstrate, denote, dim, draft,
   drop a hint, embody, exemplify, figure, fog, forebode, forecast,
   foreshadow, foreshow, foretell, give a hint, give the cue,
   glance at, gloom, have a hunch, have an intimation, hint, hint at,
   illustrate, image, impersonate, imply, incarnate, indicate,
   insinuate, intimate, lower, mean, menace, mirror, mist, murk,
   obfuscate, omen, outline, overcast, personate, personify, portend,
   predict, prefigure, presage, pretypify, prognosticate, project,
   prompt, prophesy, realize, reflect, rough out, shadow,
   shadow forth, signify, skeleton, suggest, symbolize, threaten,
   typify, vaticinate