"Hack" is a slang name for a taxi driver in general not necessarily a gypsy cab. Livery cab might be the term you're looking for.
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?