Foolish fire.

A light that misleads; a name given to a light that sometimes appears at night, usually over marshes, probably because of the combusion of marsh (methane) gas resulting from decaying vegetable matter. Other terms for it are jack-o'-lantern and will-o'-the-wisp. Anyone who attempts to follow this kind of light is misled; hence, the meaning will-o'-the-wisp.

This expression refers to a false hope, an illusion, any misleading or deluding goal, or a vain hope.

1. Literally, "foolish fire".
2. Phosphorescent light seen hovering or flitting over swamps at night, will-o'-the wisp; possibility caused by spontaneous combustion of gases given off by rotting organic matter.
3. Something that deludes or misleads; illusion, delusion.