The word "NMSL" is a pinyin(Chinese typing/pronunciation system) abbreviation of the Chinese phrase "你妈死了", which translates to "You mother died". The word is used as an insult mainly online and is a part of modern Chinese internet culture. The word gained popularity when mainland Chinese people, mostly teens, circulated government censorship to post pro-China propaganda on western social media and used this word to insult people who said anything negative about China
A(In a post): #StandwithHongkong
B(in comments): NMSL
Means your mom is dead if followed by a red heart emoji, means 'never mind the scandal and libel' against certain people.
Related to:
1. Cai Xukun fans, when Chinese people use NMSL on Cai Xukun and his fans, it becomes 'never mind the scandal and libel', just to cheer him up. Because he is really good at singing, dancing, rapping, and playing basketball, music~.
2. Andrew Bogut, an Australian center, Chinese basketball fans use NMSL on him because he is kind of a dickhead.