Shanks lost his arm while saving Luffy from a massive sea monster called the Lord of the Coast. In the first chapter of One Piece , Luffy is kidnapped by bandits and taken out to sea, where the monster attacks. Shanks rushes in to protect Luffy, and the sea monster bites off his left arm in the process. Despite this injury, Shanks uses his powerful Conqueror's Haki to scare the monster away and save Luffy

. This event is significant because Shanks sacrificed his arm for the new generation, specifically to protect Luffy, who would go on to become a great pirate. Shanks himself has said that he lost his arm betting on the future, implying he understood the importance of Luffy's destiny

. There are also theories suggesting deeper reasons behind the loss of his arm, such as Shanks having a tattoo related to the Celestial Dragons that allowed unwanted teleportation, and he may have sacrificed his arm to prevent being controlled by them. However, these are speculative and not confirmed in the story

. Additionally, the creator of One Piece , Eiichiro Oda, originally did not plan for Shanks to lose his arm. The idea was introduced later on the advice of Oda's editor to add more emotional impact and depth to Shanks' character

. In summary, Shanks lost his arm protecting Luffy from the Lord of the Coast sea monster, marking a pivotal moment that shaped Luffy's future and established Shanks as a self-sacrificing figure in the series