how does luffy get his scar

Monkey D. Luffy gets his distinctive scars from two separate incidents. The small scar under his left eye is self-inflicted; he stabbed himself to prove his toughness to Shanks and his pirate crew when he was a child. The large X-shaped scar on his chest was caused during the Marineford War arc when Admiral Akainu, using his magma powers, burned him while Luffy was unconscious. This happened when Jinbe was protecting Luffy and was hit by Akainu's attack, which also left the imprint on Luffy's chest. This scar is a reminder of Luffy's near-fatal encounter and the loss of his brother Ace in that war.