"will o' the wisp, jack-o-lantern," 1560s, Medieval Latin, literally "foolish fire;" see igneous + fatuous. "It seems to have been formerly a common phenomenon; but is now exceedingly rare" [OED].
-ify
igloo
Ignatius
igneo-
igneous
ignis fatuus
ignitable
ignite
ignition
ignivomous
ignoble
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