I don't know if I've heard pig ignorant before.
Abysmal comes from abyss, which means an immeasurably deep place (at least poetically, since some abysses that were previously immeasurable are now measurable). So I think abysmal works figuratively in that sense. If you have a scale of ignorance in the form of, say, a very tall pole that comes from the bottomless depths (the least amount of knowledge) to high above your head (the most knowledge) his level on that scale is somewhere in those bottomless depths - too far down to measure accurately. In a practical sense, it means extreme, of course.