Monkey D. Luffy, the protagonist of the popular manga and anime series One Piece, is known for his unyielding determination, devil fruit powers, and a unique scar under his left eye. The scar, which is a distinctive feature of his appearance, has intrigued fans for years. In this answer, I will explore the origins of Luffy’s scar and the events that led to its creation.

Luffy’s scar first appeared in the series during the “Sabaody Archipelago” arc, which is set after the events of the “Thriller Bark” arc. In this arc, Luffy and his crew are on their way to the Fish-Man Island to reunite with their friend and shipwright, Franky. However, they are intercepted by a group of powerful pirates called the Eleven Supernovas, who are also on their way to the same location.
One of the Supernovas, Scratchmen Apoo, attacks Luffy and his crew with his sound-based powers. During the ensuing battle, Luffy is caught off guard by Apoo’s attacks and sustains a deep gash under his left eye. The injury is severe enough that it leaves a permanent scar on his face.
However, this is not the first time that Luffy has been injured during his adventures. In fact, his body is covered with various scars and bruises, which serve as a testament to the dangers he has faced and the battles he has won. But the scar under his eye is different because it is not just a physical injury, but also a symbol of his growth and maturity as a pirate.
Before the creation of the scar, Luffy was a carefree and reckless pirate who relied on his strength and devil fruit powers to overcome his enemies. He had no real plan or strategy for achieving his goals and often acted impulsively, putting himself and his crew in danger. However, after sustaining the injury, Luffy begins to take his role as a captain more seriously and develops a more tactical and strategic approach to his battles.
The scar serves as a reminder of this change in Luffy’s character and his growth as a leader. It is also a symbol of his determination and resilience, as he continues to fight and overcome his enemies despite the injury.
In conclusion, Luffy’s scar under his eye is a result of his battle with Scratchmen Apoo during the “Sabaody Archipelago” arc. The injury is significant not just because it is a physical scar, but also because it represents a turning point in Luffy’s character development. It serves as a reminder of his growth as a captain and his determination to overcome any obstacle in his way.