Webster’s Revised Unabridged Dictionary · 1913
Dad
Dad (dăd), n. [Prob. of Celtic origin; cf. Ir. daid, Gael. daidein, W. tad, OL. tata, Gr. τάτα, τέττα, Skr. tāta.] Father; — a word sometimes used by children.
I was never so bethumped with words, Since I first called my brother's father dad. Shak.