lunatic

For centuries, people looked at a full moon and suspected it was tampering with the human brain. Latin gave this suspicion a neat label: lūna, “moon,” became lunaticus, “moon-struck,” as though madness arrived on a lunar timetable. The idea was not merely poetic; English once had the vivid expression “moon-sick,” and the supposed link between moon phases and seizures or insanity lingered in medical folklore. The same ancient family reaches unexpectedly into words connected with light: Latin lūna is traced to the Indo-European roots *leuksna- and *leuk-, “light” or “brightness,” cousins of words associated with luminous things. In 1913, Theodore Roosevelt popularized “lunatic fringe” for fanatics discrediting reform movements, turning an old medical superstition into a political insult. So when someone calls an idea lunatic, they are hurling a little piece of moonlight—and a very old fear of the night—into the argument.