Luffy has a large X-shaped scar on his chest because he was burned by Admiral Akainu during the Marineford War arc. This happened while Luffy was unconscious and being protected by Jinbe, who was carrying him away from the battlefield. Akainu's magma attack pierced Jinbe, reaching Luffy and leaving the significant scar as a result. This scar symbolizes one of Luffy's greatest near-fatal moments and reminds him of the tragedy of his brother Ace's death in the war.
