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: Webster's Revised Unabridged Dictionary (1913)