Nami loses a bet, and must adopt a duty that she rather dislikes.
The tradition had begun back in the days when it was just the five from East Blue aboard the boat. Nami had gone up on deck one day, called them all together, and declared that Goddamit, if she was going to live in close quarters with four boys, they were at least not going to be smelly ones. There was some opposition. There was an argument. When the smoke at last cleared, they had reached an agreement: one day a week would be declared Bath Day, and on that day the crew members would take turns washing themselves in a tub of warm water. Soap is mandatory.
It was a workable system. It kept the worst of the smell away without requiring too much effort on Luffy's part. Nami had to take steps to keep the others-especially Sanji-from peeking in on her, but she would have had to do that anyways. Where the problems came in was with the crew's Devil Fruit users.
When a Devil Fruit user is immersed in water, they lose all strength in their body until removed. The Straw hat Pirates quickly discovered that the same principle applied to bathwater as well. Robin was fine; She'd been a Devil Fruit user for more than twenty years, and had spent most of that time alone. She was used to giving herself sponge baths. The other two required help. Chopper was so childlike and inhuman(and so very very shy and cooperative) that he was no problem, but Luffy...Luffy was a problem.
Every bath day, when it was Luffy's turn in the tub, the other six Straw Hat Pirates would go to the wall where the schedule was posted. They would inspect it and determine which of them was stuck with Luffy Duty that week. They would then hand the poor, unfortunate soul the washcloth, and he or she would grimace, turn about and go about the unhappy task of getting Luffy clean.
"Surely this is a joke," Nami said as they clustered around the duty list. "I just did this!"
"Remember, Nami?" Chopper asked, "You bet Luffy Duty with me on whether that Sea king would get away!"
Nami groaned. She did remember that. That bet had seemed like such a sure thing! How was she to know that the smaller Sea King had friends?
"All right," Nami said as she grudgingly grabbed the washcloth out of Usopp's hand. She turned and walked out of the room, but at the door turned towards them once more.
"Oh, Sanji-kun!" she called out, injecting as much sugar into her voice as possible.
"I feel like having venison tonight."
"Aye, Nami-swan! Right away!"
Nami turned and walked away without bothering to watch the catastrophe that she had set into motion.
"Alright, Luffy," she said, grabbing him by the ear and pulling him towards the curtained corner dubbed 'The bath room', "let's get this over with." Luffy followed her, repeating "ow, ow, ow" over and over. When they reached their destination, she released him and covered her face.
"OK, Luffy, bath time. Drop'em."
After a moment the rustling sounds stopped and she removed her hand from her eyes, hoping against hope that for once-just this once!-he had thought to make at least some token effort at covering himself. As always, she was disappointed.
Reminding herself that this was, in the original conception if nothing else, all her idea, she wet the cloth and, taking great efforts to look at Luffy as little as possible, set about her duty.