One Piece Chapter 2, page 18: THEY CALL HIM "STRAW HAT LUFFY"

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.