Luffy meets Boa Hancock in One Piece episode 411 with the title “The Secret Hidden on Their Backs – Luffy Encounters the Snake Princess”
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Sometime after landing on the island “Amazon lily”, Luffy encountered its inhabitants which turned out to be all women. He later woke up finding himself placed in confinement because he was a man and men were not allowed on the island. He soon escaped and was hunted by the woman inhabitants of the island.
At that same time Boa Hancock the island’s leader was returning to the island from a call from the navy to tell her that she has been summoned to stand in defense of the navy in its seemingly upcoming battle. When she returned to her palace she decided to take a bath. At that moment Luffy was atop the roof of the palace and fell through the top straight into the bath. He sees a symbol on Boa Hancock’s back that caught his attention, but somehow this snake princess is offended and tries to use her devil fruit powers on luffy to turn him into stone.