The Collaborative International Dictionary of English v.0.48:

Daddy \Dad"dy\, n.
   Diminutive of Dad. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

daddy
    n 1: an informal term for a father; probably derived from baby
         talk [syn: dad, dada, daddy, pa, papa, pappa,
         pop]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "daddy":
   abba, dad, father, foster father, genitor, governor, old man, pa,
   pap, papa, pappy, pater, paterfamilias, patriarch, pop, pops, sire,
   stepfather, the old man