It means a white mans head is going down to suck another mans pecker. But sometimes used by when saying good bye.
I was hanging out at Conner and willies the other night, and let me tel you what, WHITE MAN DOWN! I got some that night.
Man 1: Alright man I better go home
Man 2: alright. White Man Down!
Man 1: White man down!
Man 1: Alright man I better go home
Man 2: alright. White Man Down!
Man 1: White man down!
by Wide legged trucker October 13, 2019