One Piece Chapter 2, page 5: THEY CALL HIM "STRAW HAT LUFFY"

As the pirates prepare to open the barrel, it suddenly bursts apart with a massive explosion of wood. Monkey D. Luffy pops out, stretching his arms wide and shouting, "THAT WAS A GREAT NAP!!!" The pirates are thrown back in absolute shock, crying, "WHAT THE DEVIL!?" Luffy laughs heartily, "I GOT SO DIZZY I THOUGHT I WAS GONNA BARF!! HAHAHAHA!" He looks down at the broken pieces of the barrel and remarks, "HMM... LOOKS LIKE I SURVIVED SOMEHOW."