How can Nami's punches hurt Luffy so much?
Here's another answer, also on SBS

D: Blows shouldn't hurt Luffy, so why does he get bruises and swelling when Nami and the others beat the crap out of him?! Tell me!!! P.N. Zororin Love
O: When the crew beats up on each other, it's always in admonishment with a strong sense of purpose, which is why it works so well!! It works on his spirit!! I bet Nami is thinking, &quot;It's his spirit that hurts deep down!!&quot; ...right? I guess not.

So Nami and others didn't punch Luffy physically, but they hurt his spirit, and with doing so, his body also get hurts

right? I guess not.

I think Oda doesn't really want to give any logical answer here