It's possible I've heard this one before, but I like it too much to pass it up, and I don't think I ever knew what it meant before. A "cacoëthes" is a strong desire or urge. Derives from a Greek word meaning "wickedness." Apparently one of the most common modern uses is in the phrase "cacoëthes scribendi," usually referring to an insatiable and/or irresistible urge to write; often specifically in reference to an Oliver Wendell Holmes poem titled "Cacoëthes Scribendi."