The slang term "wak" is used frequently in the Hip-Hop subculture to describe something which is poor, substandard, uncreative, or just crappy in general. The word apparently stems from the phrase "out of whack". Usage of "wak" can be found as far back as the late 1970s, when the Hip-Hop subculture was in its infancy.

Although variations on the word such as "wack" and "whack" exist, the preferred spelling in the Hip-Hop community is "wak"

Sources:
Henry Chalfant and Martha Cooper. Subway Art. New York: Henry Holt and Company, 1984.
Henry Chalfant and James Prigoff. Spraycan Art. New York: Thames and Hudson, 1987.