Meaning of "on the wax" from an article in Stanford Encyclopedia of Philosophy
This is obviously a reference to the wax tablets that were used for drafts and note-taking before the invention of paper; after you had made a fair copy onto a roll of papyrus, you could simply smooth over the wax tablet and re-use it. Nowadays we would probably say 'in manuscript', but in ancient times, of course, all writing was in manuscript, so apparently the Stanford author thought this would be a clever alternative.