The Collaborative International Dictionary of English v.0.48:

Nabob \Na"bob\ (n[=a]"b[o^]b), n. [Hind. naw[=a]b, from Ar.
   naw[=a]b, pl. of n[=a]["i]b a vicegerent, governor. Cf
   Nawab.]
   1. A deputy or viceroy in India; a governor of a province of
      the ancient Mogul empire.
      [1913 Webster]

   2. One who returns to Europe from the East with immense
      riches: hence, any man of great wealth. "A bilious old
      nabob." --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nabob
    n 1: a governor in India during the Mogul empire [syn: nawab,
         nabob]
    2: a wealthy man (especially one who made his fortune in the
       Orient)