[ig-neez fach-oo-ahy] /ˈɪg niz ˈfætʃ uˌaɪ/ (Show IPA)
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.
something deluding or misleading.
Origin of ignis fatuus
1555-65; < Medieval Latin: literally, foolish fire