We should use this word for bad handwriting or bad spelling more often, it’s too useful and relevant to let it fade away.
It derives from the Greek graphos, “writing”, prefixed with kakos, “bad”. We’re more familiar with this as the beginning of cacophony, “bad noises” (despite the association of ideas, it has nothing to do with our cack-handed, which derives from Old English cack, “excrement”).
When cacography began to appear in English at the end of the sixteenth century it did so with the sense of “bad spelling”. It was beginning to be thought that the old way of spelling words by personal preference ought to give way to a standardised system; the introduction of printing had a lot to do with this. So cacography was seen as the opposite of orthography, “correct spelling”. In the following century cacography was used to mean bad handwriting as well, as the opposite of yet a third Greek word, calligraphy, “fine writing”.
The word is marked as archaic in my dictionaries, though it still turns up from time to time. A typical usage was that by the horror writer H P Lovecraft, who described the manuscript of his novel Quebeck as “136 pages of crabbed cacography” (in reference presumably to the handwriting rather than the spelling).
Someone who exhibits either failing is a cacographer.