Note!! There will be Heavy spoilers.
Since the introduction of Haki (after the 2 year time skip), the colour of arms Haki has been the main way of hitting and damaging most defenses from devil fruits (Paramecia and logia type). But in Kaido's case, hitting him with Haki (even when he isn't using Haki) does no damage to him.
How is it possible that even the Color of Arms Aka Busoshoku Haki didn't seem to phase him at all. Is this related to the Tekkai technique used by the Cp-9? (We know the world government captured him at some point). Or is this ability an effect of Kaidos "serpentine dragon" devil fruit.
Also is it similar to Big Moms "invincibility"? Because I also remember Bege talking about how Big Mom had never been wounded till the time she cried from Mother Carmel's portrait.