What is the deal with some Emperors 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.
In the One Piece world, people can have pretty different biologies, some having special traits that are rather overpowered. A good example of how this is "genetical" is the Funk brothers; Kelly was beaten by one punch from Boo, but when Boo used an axe infused with Armament Haki against Bobby it BROKE against his BACK!! Bobby is said to have never actually fought in his life, and I doubt that he would just happen to be skilled at Haki. Kelly specifically mentions that Bobby was born with such a strong body.

Big Mom is the same, ever since childhood she has been practically invulnerable. This might actually be a sort of subconsious Haki, but it's more likely that it's just the way her body is.

And before anyone tells me "bodies don't work that way, getting stabbed by a sword without haki protecting you will always hurt you", this is One Piece. If Luffy could send a 20+ meter long jaguar flying with a punch (Amazon Lily, before he learned any haki) then can you really claim that it's impossible? Why would super-strength be common, and super-durability impossible?