Shanks lost his left arm when he sacrificed it to save Luffy from being attacked by a Sea King. Despite his immense strength and mastery of Haki, Shanks chose to let the Sea King bite off his arm to protect Luffy, who had been kidnapped and was in danger of drowning. This sacrifice was symbolic of betting on the future and the new era that Luffy and his generation represent. Additionally, recent revelations suggest that Shanks may have had a deeper, more complex reason involving a special mark or lineage that made sacrificing his arm necessary for severing ties with his past or protecting Luffy's destiny. The act of losing his arm was both a physical sacrifice and a pivotal moment that deeply influenced Luffy's path to becoming a pirate.
In summary:
- Shanks lost his arm protecting Luffy from a Sea King attack.
- The sacrifice was symbolic of his belief in the new generation.
- New story details hint at a possible connection to a mysterious mark on Shanks' arm and his lineage, adding complexity to why he allowed his arm to be bitten off.
