Why did men in Song Dynasty take the word "Lang" when they named their names? Such as Liu Guo, line eighteen lang.

it's not that men in the song dynasty were named with "lang", but they were nicknames of their family and friends before they took the word

for example: 1. Wu Dalang, whose name is actually "Wu Zhi", is just a title, not "lang" or "Yang Sanlang", that is, Yang Gao. In the Northern Song Dynasty, he was a general in front of the temple, with remarkable military exploits and loyal military commanders.

PS: It is very similar to "Eighteen Lang" mentioned by LL. Perhaps Liu Guo is the 18th in the family (or clan), so "Walking Eighteen Lang"

is a reference from Baidu Encyclopedia.