recorded from c.1500, but probably much older, from child's speech, nearly universal and probably prehistoric (cf. Welsh tad, Irish daid, Czech, Latin, Greek tata, Lithuanian tete, Sanskrit tatah, all of the same meaning).
Slang definitions & phrases for dad
dad
noun
An old man Used as a disrespectful term of address toward older men. The similar term pop is not similarly disrespectful: Okay, dad, outta the way and you won't get hurt(1950s+)
God Used euphemistically as an element in various old-fashioned mild oaths like dad-blamed (1670s+)