One Piece Chapter 2, page 17-19: THEY CALL HIM "STRAW HAT LUFFY"

Alvida swings her spiked iron mace with full force, screaming, "YOU'RE BOTH GOING TO DIE!!" Koby cries out, "L-LUFFY...!!" but Luffy steps in front of him, smiling, "GOOD FOR YOU, KOBY!! IT MAKES NO DIFFERENCE TO ME..." The mace strikes Luffy directly on the head with a heavy "WHAM!!" impact. However, Luffy remains completely unharmed, smiling widely as he says, "THAT DIDN'T HURT! 'CAUSE I'M MADE OF RUBBER!" Alvida is shocked, stammering, "IMPOSSIBLE! NO ONE SURVIVES THE IRON MACE!!

Luffy stretches his arm far back, preparing an attack. Koby and the pirates watch in utter disbelief, stammering, "YOU'VE GOTTA BE KIDDING!" and "WHAT THE--?" Luffy yells, "GUM-GUM......PISTOL!" and unleashes a powerful stretched punch that hits Alvida squarely in the stomach with a massive "WOM!!!" sound effect, sending her flying through the air.

Alvida lies defeated on the ground. Her crew is terrified, crying, "HE BEAT LADY ALVIDA! HE'S NOT HUMAN! CAPTAIN!!!" Luffy turns to the remaining pirates and commands, "GIVE KOBY A DINGHY! HE'S GONNA JOIN THE NAVY!" The pirates quickly salute, stammering, "Y-YESSIR..." Luffy laughs, "HA HA HA!" while Koby sheds tears of gratitude, whispering, "LUFFY...