"Nigga" is a slur for black people. Often referred as the N-Word or N-Slur. Shouldn't be used in a sentence. If you see someone use it, they are probably racist.
Person 1: "Hey man, remember that nigga from the-"
Person 2: "Did you just say the N-Slur? You're racist!"
WML means "wish me luck".
W - Wish
M - Me
L - Luck
Person 1: okay guys, I'm gonna do that skateboard trick, wml!
Person 2: good luck, bro!
Person 1: "A word."
Person 2: "Do you mean ass?"
Person 1: "No, literally a word."