Hack specifically refers to gypsy cabs in many places esp. Baltimore: http://articles.baltimoresun.com/2009-09-15/news/0909150013_...
I understand that cab drivers have been called hacks, and that of course the term's origin is more general than its current usage. I hear "hack license" but I never hear "hack" as a verb except to refer to doing it illegally.
edit: or as an anything but something before the word "license." You've heard a legitimate taxi driver called a hack lately?