Etymology
1. From Early Modern English dadd, dadde (circa 1500), undoubtedly older, from unrecorded Middle English *dadde, of uncertain ultimate origin. Compare West Frisian deite (“dad, daddy”), Swabian Dede (“Godfather”).
2. * Perhaps of Celtic origin, compare Welsh and Breton tad (from Proto-Brythonic *tad), Old Irish data; and possibly related to Russian дя́дя (djádja, “uncle”) and/or Russian де́душка (déduška, “grandfather”), all imitative. In Welsh, when subject to soft mutation (which occurs in vocative contexts, among others), tad becomes dad.
3. * Perhaps imitative of a child's first uttered syllables da, da.
4. * Possibly from a metathetic variation of a hypothetical Old English *ætta, *atta (“father”), from Proto-West Germanic *attō, from Proto-Germanic *attô ("father, forefather"; whence also North Frisian ate, aatj, taatje, tääte (“father; dad”), Middle High German tate (“father, dad”) (whence German Tate (“dad”), Bavarian tatte (“dad”), Cimbrian tatta (“dad”)), Icelandic táta (“dad”)), from Proto-Indo-European *átta (“father”), whence Sanskrit तत (tata, “father”). See: Frisian, Northern 'ate', Icelandic '
táta'.