Can also be spelt 'wack'.
Leroy: Those shoes are whack! You look like a douche.
Logan: Fuck you. Shut yo whack ass up.
Leroy: Seriously they are whack!
Whilst whack is also a verb that means to hit (or kill), it has been used as an adjective since the 80s, increasing in use more recently. It can be used to describe any number of items but particularly songs, movies and clothes.
In the song 'Life Goes On' by 8 Ball & MJG ft Slim Thug, Slim Thug's line: Radio only play shit that's whack. Here's a clip of the song: