From WordNet (r) 3.0 (2006) [wn]:
daddy
n 1: an informal term for a father; probably derived from baby
talk [syn: dad, dada, daddy, pa, papa, pappa,
pop]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Daddy \Dad"dy\, n.
Diminutive of Dad. --Dryden.
[1913 Webster]