“Happiness Punch” is a joke technique that Nami uses to dispatch people watching her bathe; she simply drops her towel, causing any male in the vicinity to collapse from a massive nosebleed. She also uses this as a get-rich-quick scheme, as she charges 100,000 Berries to anyone who sees her doing this.
In SBS Volume 74 Oda replied to a fan who asked how many people can Nami’s Happiness Punch take out.
Question: Luffy’s Haoshoku Haki can take out 50,000 people, but how many people can Nami’s Happiness Punch take out? P.N. Captain Nobuo
Oda: Oho–. I see. This is tough showdown, isn’t it. Well let’s see, if we were to talk about these two using their techniques with their own bodies alone, Nami’s Happiness Punch would only work among “visible range”, so it would probably not be able to compete against Luffy’s Haki. However, if we are allowed to take the usage of Visual Den Den Mushi into consideration, Luffy’s Haki will not be able to convert into willpower-radio waves, but Nami will be able to attack various locations around the world at once, and thus victory would become Nami’s. Nevertheless, it should be noted that the “Happiness Punch” also deals a great amount damage to one’s wallet as it requires the payment of 100,000 berries per person, and in this way, it can be confirmed that this attack is a great deal nastier than Haki in general.