I'm not entirely sure which 'arm fall off boy' you specifically refer to. It could be a very specific or niche character from a certain comic, game, or local story. Without more context, it's difficult to detail the origin story.
If this is a relatively unknown or new character, it might be from a new indie comic or an online fan - created story. In some cases, such characters are born out of the need to explore different body horror or surreal concepts. The origin story could involve a scientific experiment gone wrong in the fictional world, or perhaps a curse that was placed on the boy, causing his arm to have this strange property.
Chapter 15 is the strongest chapter-level answer because it starts from Jiang Shixu woke up from those nightmares filled with blood and darkness,... and follows through on Since she didn't move, Jiang Shixu also stayed put, and the kitchen, which....
Chapter 635, "Forced Return to the Time-Space Administration", is the clearest early answer because it pushes Jiang Li into a new phase or setting. The chapter gives readers a real answer by opening on But Pei Ye was much quicker than she had imagined.
Chapter 897, "Are You Worried About Me?", is the clearest early answer because it turns Chang Rong's situation into a concrete plot event. The chapter gives readers a real answer by opening on Changqing stood at the roadside with the night breeze blowing,...
Chapter 671, "Who Understands Family!", is the clearest early answer because it turns Gu Yu's situation into a concrete plot event. Early on, Director Meng drove the shuttle bus around Jingcheng Airport.. By the end, Heard he was also taking media as a minor in adult college, and his....
Chapter 863, "Your Marriage is a Calamity", is the clearest early answer because a shaky connection becomes usable in the plot. Early on, The raffle results came out quickly.. By the end, My fiancé is thrilled too; we were planning to get married next year,....
Chapter 868, "The Story of the Old Sea King Coming Ashore", is the clearest early answer because it turns Xue Wandeng's situation into a concrete plot event.
Chapter 770, "You're Actually Speaking Up for Her?!", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Back in Chixia Village, Jiang Li had already begun showing utter...
Chapter 916, "Not Biological?!", is the clearest early answer because it turns Meng Xuezhi's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Bang," Hong Pei slammed the teacup onto the table.
Chapter 899, "It's actually a seal script?!", is the clearest early answer because it turns Hua University's situation into a concrete plot event.
Chapter 850, "Wanted to Recognize Her as a God-Granddaughter", is the clearest early answer because it turns Rong's situation into a concrete plot event. Early on, Though Jiang Li might guess her family situation, he would never know....