The plum tree is on the road, the plum nobody wants. It must be bitter.
世說新語(Shishuo Xinyu; A New Account of the Tales of the World) by 劉義慶 (Liu Yiqing). It is a historical compilation of many Chinese scholars, musicians, and artists during the 2nd-4th centuries. It contains many idioms which are still commonly used in chinese character culture boundaries. This story is about 道傍苦李; Bitter plums on the road. Wang long(王戎) is from Jin(晉), the famous retainer.
When Wang long was young he and the other children found a plum tree on the road. The plum tree had lots of plums, so the other children went for those. But Wang long was just looking at the other children. Someone asked Wang long, why wouldn't he go for the plums. He answered “The tree is on the road and still so many fruits left, the plums must be bitter.” Later they ate the plum and the plums were indeed bitter.
I guess Wen kexing is saying the 琉璃甲(liulijia; glass lock) is the bitter plum. He is laughing at the jianghu people, because they are all running for that bitter plum. I wonder how Wen kexing knew what Zhou zishu thought. Can he hear the others inner voice?? I see some cross-over here 🤭