The Collaborative International Dictionary of English v.0.48:

Unimpressed \Unimpressed\
   See impressed.

WordNet (r) 3.0 (2006):

unimpressed
    adj 1: not moved to serious regard; "trying to appear
           unimpressed"

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "unimpressed":
   aweless, calm, composed, cool, expected, expecting, inexcitable,
   unaffected, unamazed, unanimated, unastonished, unastounded,
   unawed, unbewildered, undazed, undazzled, undumbfounded,
   uninspired, unmarveling, unmoved, unruffled, unstirred, unstruck,
   unsurprised, untouched, unwondering, wonderless