The Collaborative International Dictionary of English v.0.48:

Dad \Dad\ (d[a^]d), n. [Prob. of Celtic origin; cf. Ir. daid,
   Gael. daidein, W. tad, OL. tata, Gr. ta`ta, te`tta, Skr.
   t[=a]ta.]
   Father; -- a word sometimes used by children.
   [1913 Webster]

         I was never so bethumped with words,
         Since I first called my brother's father dad. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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