Surely you know that cacophony is "harsh sound." In that same vein, cacography is both "bad spelling" or "bad handwriting." I take this to mean that when you attempt to sit down and read something border on chicken-scratch illegibility, your head begins to hurt... like your reaction to hearing the classic horror-sound of fingernails a-clawing on a chalk board. In other words, harshness of writing. When I come across passages in a novel that could have been better edited, I cringe in the same way.