Source : Webster's Revised Unabridged Dictionary (1913)

Dad \Dad\, n. [Prob. of Celtic origin; cf. Ir. daid, Gael.
   daidein, W. tad, OL. ?, ?, Skr. t[=a]ta.]
   Father; -- a word sometimes used by children.

         I was never so bethumped withwords, Since I first
         called my brother's father dad.          --Shak.

Source : WordNet®

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