Webster's Revised Unabridged Dictionary · 1913

Dad

Dad (dad) , noun

[Probably of Celtic origin; compare Ir. daid, Gael. daidein, Welsh tad, Old Latin tata, Greek ta`ta, te`tta, Sanskrit tāta.]

Father; -- a word sometimes used by children.
I was never so bethumped with words, Since I first called my brother's father dad. — Shakespeare