When a white person is hanging out with any amount of black people and drops the word nigga. They immediately stiffen up and stay as still as possible in the hopes that the black people won't recognize they said the word nigga.
Some random white dude says nigga to a bunch of black guys, then the nigga mortis occurs.
The death of a black man. Initially the name of the album "6 Feet Deep" by Gravediggaz, but they had to rename it due to the whiney cunts that found it offensive.
The posture/reaction of a black person when they see a police officer. Niggamortis occurs in just .1 of a second and in this time they decide if they should play it cool or run from saidlaw enforcement.