The Collaborative International Dictionary of English v.0.48:

Overshadow \O`ver*shad"ow\, v. t. [imp. & p. p. Overshadowed;
   p. pr. & vb. n. Overshadowing. ] [Cf. Overshade. ]
   1. To throw a shadow, or shade, over; to darken; to obscure.
      [1913 Webster]

            There was a cloud that overshadowed them. --Mark ix.
                                                  7.
      [1913 Webster]

   2. Fig.: To cover with a superior influence; to be viewed as
      more important than. --Milton.
      [1913 Webster +PJC]

   3. To cause to be sad or disappointing; to cast a sad shadow
      on; as, an accidental death overshadowed the joy of the
      festival.
      [PJC]

WordNet (r) 3.0 (2006):

overshadow
    v 1: be greater in significance than; "the tragedy overshadowed
         the couple's happiness" [syn: overshadow, dominate,
         eclipse]
    2: make appear small by comparison; "This year's debt dwarfs
       that of last year" [syn: shadow, overshadow, dwarf]
    3: cast a shadow upon; "The tall tree overshadowed the house"

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "overshadow":
   adumbrate, becloud, bedarken, bedim, befog, begloom, bemist,
   bestraddle, bestride, black, black out, blacken, blight,
   block the light, blot out, brown, cast a shadow, clabber up, clear,
   cloud, cloud over, cloud up, command, cover, curtain, darken,
   darken over, dim, dim out, diminish, dominate, draw the curtains,
   dwarf, eclipse, encloud, encompass with shadow, enmist, excel,
   extinguish, fake out, fog, gloom, haze, impair, look down upon,
   mar, minimize, mist, murk, nubilate, obfuscate, obnubilate,
   obscure, obumbrate, occult, occultate, outshine, outtop, overarch,
   overcast, overcloud, overlook, oversmoke, overtop, put down,
   put to shame, rise above, ruin, screen, shade, shadow, show up,
   shutter, smog, smoke, somber, spoil, surmount, top, tower above,
   tower over, veil