The Collaborative International Dictionary of English v.0.48:

mind-boggling \mind-boggling\ adj.
   intellectually or emotionally overwhelming; straining one's
   capacity to comprehend or cope; as, a mind-boggling display;
   a mind-boggling puzzle.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mind-boggling
    adj 1: intellectually or emotionally overwhelming; "a mind-
           boggling display"; "a mind-boggling puzzle"