Shanks lost his left arm while saving a young Luffy from a massive sea monster known as the Lord of the Coast. In the first chapter of One Piece , Luffy was kidnapped by bandits and taken out to sea, where the monster attacked. Shanks rushed to protect Luffy, and during the confrontation, the Lord of the Coast bit off Shanks' arm. Despite the injury, Shanks used his powerful Conqueror's Haki to scare the monster away and save Luffy

. Shanks himself explains that he sacrificed his arm for the future-the new generation-implying he saw potential in Luffy and wanted to protect him at all costs. This act deeply influenced Luffy's dream of becoming a great pirate. The loss of his arm also humanized Shanks, showing his willingness to sacrifice for others despite his immense strength. Additionally, the injury affected his rivalry with Dracule Mihawk, as Mihawk no longer wished to fight Shanks knowing he was at a disadvantage due to the missing arm

. There are fan theories suggesting hidden reasons behind the loss, such as Shanks having a secret tattoo or lineage-related significance, but the canonical explanation remains that he lost his arm protecting Luffy from the sea monster

. The author Eiichiro Oda originally did not plan for Shanks to lose his arm, but his editor suggested it to add emotional weight to the story

. In summary, Shanks lost his arm by sacrificing it to save Luffy from a sea monster attack early in the series, an event that shaped both characters' futures profoundly.