Hancock answers a few questions for him.

(Okay, going to pull a bit more of sexyslave's concepts for this one, lol.)

"Eh? Why did your tail return?" Luffy asked Hancock, noticing that her legs were gone again, and the tail, attachment to the lamp spout and all, were in their place.

"Oh, that, Master?" Hancock tittered. "Like I said, I need your permission to have legs, this is my default state now without said permission. You wanted me to have legs so we could do the deed, but because that's over, your permission expired... This tail is a physical representation of my bondage to the lamp. And it's also a sexy chastity belt, can't have my pussy without your permission either~!" she gushed, clearly turned on by her situation.

"...oh... Wait, can't you just move the lamp around? You can fly now, right?" Luffy asked.

"Not that far from it, Master," Hancock denied. "And no, I can't carry the lamp. Genies are literally physically incapable of moving their lamps. Again, symbol of bondage."

"...you just need to eat more meat then, then you can get strong enough to lift it!" Luffy suggested, his simple mind shining through.

Hancock laughed at the suggestion. "It doesn't work like that, Master!" she giggled.

"Why not?" Luffy pouted. "Seriously, what kind of lamp is it anyway?" he asked, going to pick it back up.

"It's an entity of sorts... It comes from dimensions well beyond ours, and feeds off of metaphysical energy produced by the fulfillment of sexual desires. And I am an extension of it now. Still Hancock, but also more than what I was before."

"So... It's a Mystery Lamp then!" Luffy grinned.

Hancock covered her mouth as she laughed again at his simple nature. "Sure, let's go with that," she allowed easily.