'dafux' means 'da fuck', as in: 'what dafux?'. But, if you're cool enough, you can just say 'dafux'.
"yo homedizzle, dafux is up?"
"nothin' be goin on in dis heazy, homie. we all be chillaxin in heah!"
A short version of what the f*ck?
What dafux is going on?