ignis fatuus [ig-nis fach-oo-uh s]
noun, plural ignes fatui [ig-neez fach-oo-ahy]
1. Also called friar's lantern, will-o'-the-wisp. a flitting phosphorescent light seen at night, chiefly over marshy ground, and believed to be due to spontaneous combustion of gas from decomposed organic matter.
2. something deluding or misleading.
Origin: 1555-65; < Medieval Latin: literally, foolish fire
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