“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.
In SBS Volume 74 Oda replied to a fan who asked how many people can Nami’s Happiness Punch take out (since Luffy’s Conqueror’s Haki can take out 50,000 people):
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 belis per person, and in this way, it can be confirmed that this attack is a great deal nastier than Haki in general.”
Looks like Nami’s “Happiness Punch” hit again in chapter 936.
Drake and Hawkins’ platoon raids the Flower Capital bathhouse to arrest anyone with a crescent moon symbol on their ankle.
Hawkins commands all the bathers to line up and show him their ankles. Hawkins sees Nami hiding in the corner and orders her to come out, shooting his sword toward her.
Robin gets ready to fight if necessary. Nami steps out and tries to act innocent. Hawkins’s sword catches Nami’s towel, making it fall to the ground and exposing her naked body for all to see, which causes several patrons to break out in nosebleeds.
Nami picks up her towel when suddenly, a previously invisible Sanji in his raid suit gets a nosebleed as well and throws himself backwards from the force of the hemorrage.
The other women in the bath recognize him as Soba Mask. Nami and Robin are shocked to see him and his outfit. A henchman informs Drake of Soba Mask’s presence, Drake enters the bath and immediately faints.