From 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]