One Piece Chapter 2, page 6: THEY CALL HIM "STRAW HAT LUFFY"

Luffy looks around at Koby and the three stunned pirates, asking, "WHO ARE YOU GUYS?" The pirates scream in unison, "WHO THE HECK ARE YOU!!?" and demand, "WHAT WERE YOU DOING IN THERE!?" Suddenly, a massive iron mace flies through the air with a loud "VWOOOM" sound effect. A voice roars, "BACK TO WORK, YOU SCURVY DOGS!!!" The mace smashes into the building behind them with a colossal "KA-BLASH!!!", sending debris flying as the pirates scream, "ARRGH!"