This is a typical Chinese slang, which is the initials for “cào nǐ mā bī” which means fuck ur mom’s vagina. Often used in curses for online/real life arguments.

the thing that you don’t have when you’re phones percentage is over 20

which is the initial of "China never makes breakaway" and highlights the sovereignty of China.

CNMB = Chinese National Missile Defence

Acronym of the Chinese swear "Cao Ni Ma Bi". Literally it translates to "Fuck Your Mother's Pussy", which means (I) (will)/(want) fuck your mother or I'm your dad.

Unlike the English word of "motherfucker", which suggests incest of the object person, an user of "CNMB" shames others by flaunting itself as a (powerful) rapist/conqueror.

cnmb stands for (c)ould (n)ever (b)e (m)e