Stands for "Nigga What The Fuck!", not like the other "NWTF", "Now What The Fuck".
The phrase is used for a moment of feeling gifted(a moment of just something wrong for or something bad happened to the person), or looking at something funny or someone that just needs to quit life.
When using the phrase, don't say the word as NWTF!, but say it as NIGGA WHAT THE FUCK!!. Say it with speed and emphasis. The correct way of actually saying "NWTF" is saying "Oh man, my girl said I couldn't cut today cause she was on her period, biggest NWTF moment of the week".
Me: Hey man, listen to this song by Oj Da Juice Man.
Friend: Ight. (Three minutes later). Man that was the biggest NWTF moment ever.
Friend:You ready to fuck?
Girlfriend: Yeah... (Kid busts through the door).
Friend: NIGGA WHAT THE FUCK!