The Collaborative International Dictionary of English v.0.48:

Bigwig \Big"wig`\, n. [Big,a.+ wig.]
   A person of consequence; as, the bigwigs of society. [Jocose]
   [1913 Webster]

         In our youth we have heard him spoken of by the bigwigs
         with extreme condescension.              --Dickens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bigwig
    n 1: the most important person in a group or undertaking [syn:
         kingpin, top banana, bigwig]