Nigga is a word that originated from black people after changing the racial slur nigger into a and common word in black culture.
A lot of times this word my be substituted by the words following: "bruh" "bro" "man"
Commonly those who substitute the word are Caucasian people who just sound stupid saying nigga.
This word can be used in many instances
For example
Greeting"YO WASSUP MY NIGGA!"
Intimidating greeting "Whats good nigga?"
Used in a sad tone, usually what something doesn't go your way"Nigga damn!"
"Nigga what the fuck?!"