This has nothing to do with my novel, but I just learned that the proverb that gives this post its title dates back at least to Chaucer's Physician's Tale (c. 1390).
A theef of venysoun, that hath forlaft His likerousnesse and al his olde craft, Kan kepe a forest best of any man.
I made my way through most of The Canterbury Tales in college, but the Middle English wore me down before I reached the book's cul (so to speak).