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The term is Urdu, borrowed via Persian from the Arabic being the honorific plural of naib i.e. 'deputy'. In some areas, especially Bengal, the term is pronounced Nobab. This later variation has entered the English and other foreign languages, see below.

The title Nawab or Nawaab is basically derived from the title of the four nayab (deputies) of 12th and last Imam (Imam-e-zamana) of the Shia sect. That is why most Shia rulers have called them Nawab instead of Sultan or King.
... In response to Nitrox's querry on Nawab.

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