This is set post-Thriller Bark.
So if you really want to understand, you need to have seen the conversation between Nami and Lola....Hehe.
Nami's enraged shriek carried through the whole ship, pulling Zoro from the firm grasp of sleep. He yawned broadly, scratched his head, and hoping that her ire was not directed at him, though he's guessing that it has something to do about Nami's gender change. The gender change she wasn't aware of, till just now, it seems.
"What the hell is wrong with you?!" Nami roared, her fist crashing down on the unfortunate Usopp's head. "Of course I'm a woman! Where the hell do you get off telling people I'm a man?!"
Usopp gingerly fingered the swelling lump on the top of his head and made a mental note to have Chopper look at it later, before glaring at the fiery woman. "What was that for, Nami?" he demanded. "You told that warthog lady you were a man!"
Nami leaned across the table to beat him over the head again, reinforcing every syllable with a blow. "I-di-ot! That was a lie! You've seen me naked before! You know I'm a woman!"
She climbed even farther over the table, trying to reach the marksman who was attempting to crawl out of reach of the fearsome navigator, both of them too preoccupied to notice Zoro, who had just entered the ship's kitchen. He raised an eyebrow, but refrained from commenting, choosing instead to search the fridge for rum, grog, or whatever the ero-cook had picked up at the last port.
"It was all steamy, both times!" Usopp protested, cowering. "I couldn't hardly see anything! You could still be a man, for all I know! Maybe you just have a really small-!" He suddenly decided that he wasn't even remotely happy with that train of thought, and cringed against the wall to avoid Nami's flailing fists.
Zoro headed for the door, the rum he'd come to find in hand, shaking his head at the sheer idiocy of the situation, before finally deciding to intervene on Nami's behalf. Hell, maybe she'd reduce his debt a bit, or at least lower the exorbitant interest rate.
The sharpshooter and the navigator both paused and looked up at the green-haired swordsman.
"I can assure you that Nami is, in fact, a woman." And with that and a smirk, he exited the kitchen, leaving Nami shocked, and Usopp with plenty of fodder for a brand-new rumor.
I might post a longer version later.
Keep your eyes out, babies!