Daily Chinese Idiom Guessing Game
To stand by and watch without offering any help or taking any action.
把手笼在袖子里，在一旁观看。比喻置身事外，既不过问，也不协助别人。
唐·韩愈《祭柳子厚文》不善为斫，血指汗颜，巧匠旁观，缩手袖间。”宋·苏轼《朝辞赴定州论事状》奕棋者胜负之形，虽国工有所不尽，而袖手旁观者常尽之。”
While the two were arguing, he just stood by with his arms folded and watched.