• Etymology Today from M-W: will-o'-the-wisp \wil-uh-thuh-WISP\
1 : a light that appears at night over marshy ground; *2 : a misleading or elusive goal or hope

The will-o'-the-wisp is a flame-like phosphorescence caused by gases from decaying plants in marshy areas. In olden days, it was personified as "Will with the wisp," a sprite who carried a fleeting "wisp" of light. Foolish travelers were said to try to follow the light and be led astray into the marsh. (An 18th century fairy tale described Will as one "who bears the wispy fire to trail the swains among the mire.") The light was first known, and still also is, as "Ignis Fatuus," which in Latin means "foolish fire." Eventually, the name "will-o'-the-wisp" was extended to any impractical or unattainable goal.
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