Who is The Hack Writer from On Writing Well and what is their importance?

Zinsser occasionally mentions how some writers are hacks. He does not clearly define this term, possibly because it is writer's jargon, possibly because it is the worst thing one can call a writer outside of street curses. Considering the strong ethics that Zinsser holds, the hack writer must be one who does not follow a similar code. The hack writer fabricates sources, misquotes interview subjects, stretches the truth, promotes outright lies, and probably has not mastered the English language.