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!
when a White man who is part of a group includes women and/or minorities feels that he must assert himself by sharing (often unsolicited) knowledge regarding a subject that is being discussed. This makes him feel and potentially appear to be the authority on this subject, in that group.
This guy is married to a Karen, and unfortunately he suffers from White Man In The Room Syndrome.