Some crazy power scaling head cannon in here lol.
TCB with extra lines are ....
Since there no break hopefully with get who traitor is next week .
Yes you couple that with the fact that the WG were doing endurance tests on King, it means it is endurance not immunity or invulnerability
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"Toughness" has always been used for endurance. Kaido was said to be tough, preskip Zoro was said to be tough. The fact that Luffy likened the Sera's to Kaido also supports the endurance route.
That not all ways the case when luffy was talking about hurting Kaido he was talking purely about toughness.( like his scales )
Endurance was not a factor then since he could not get pass his defence and that why he train for a week .
Truth is the context is most important thing look at since people translate thing differnt .