One Piece fans are still wondering how Nami can beat Luffy when his body is made from rubber. Here are some answers.
It is a long-standing question within the fandom how Nami can hurt Luffy. The Straw Hat captain is made out of rubber so it shouldn’t be surprising that there’s a significant power difference. Nami has always beaten Luffy to a pulp every time she sees him. Similar questions were also asked when Luffy was beaten up by his crewmates.

There is no evidence that Luffy could be subject to these types of beatings by Nami, as far as anyone knows. That hasn’t stopped people from asking questions or trying to find out the answer. Many fans, and even Eiichiro Oda himself, have suggested ways this phenomenon can be achieved.
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What are the Possibilities for Nami To Hurt Luffy’s Health?
Technically, there is already a canon solution to this question. Oda believes that Nami is truly attacking Luffy’s spirit as much as his body. He reasoned that the beatings are usually in admonishment, with a strong feeling of purpose. This is what hurts Luffy’s spirit and hits him in his soul.
This idea of beating up a Rubber Human might have merit if you have the right convictions. Monkey D. Garp stated that he was able to beat his grandson thanks to the familial affection he felt. Perhaps Luffy hurting is a matter of the intention behind it.
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The answer to this question may also depend on the time and manner it was given. He provided this answer as part of the SBS Volume 43, while the Enies Lobby Arc still was underway. Oda didn’t know of a concept and was therefore unable to provide an answer. Even he acknowledged that he was not certain of the answer.
Haki was one such concept that was not introduced during the Enies Lobby Arc. Although clues to this power system’s existence were found scattered throughout the story, it would take several years to officially introduce it to the story. Many fans believe Nami is using an undeveloped Armament Haki to inflict pain upon her captain after the introduction of Haki.
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Another option is Luffy’s Gum-Gum Fruit. Or, as it is called, the Human-Human Fruit Model. Nika. This Mythical Zoan Fruit, in addition to giving Luffy rubber-like properties, allows him all kinds of crazy feats like jumping off the ceiling and reaching through his eyes. It makes him a cartoon character.
Could Cartoon Physics Have Anything to Do with It

These cartoon-physics applications could explain why Nami (and a few others) can beat up Luffy. These beatings can be comical, but it’s important to keep in mind that they are intended to have a humorous effect. Luffy’s powers, which are comical by their nature, may experience as much pain or as laughter.
RELATED: One-piece: Was Vegapunk York Usually Tall? Luffy’s enemies might find it difficult to bring him down by getting hurt only when it is funny. Because they take Luffy’s fight seriously, they must use traditional Haki to inflict lasting damage. This, along with slashing, stabbing, and other attacks. Nami and Luffy might not get along in a serious bout for any reason. You can always attribute these cartoonish beatings or other established story elements. They are comical effects. They don’t have any logic and are quickly forgotten. They are simply one of many running gags within One Piece, and it appears that this is all they were meant to be.