Source : Webster's Revised Unabridged Dictionary (1913)

Daddy \Dad"dy\, n.
   Diminutive of {Dad}. --Dryden.

Source : WordNet®

daddy
     n : an informal term for a father; probably derived from baby
         talk [syn: {dad}, {dada}, {pa}, {papa}, {pappa}, {pater},
          {pop}]