will o' the wisp


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will o' the wisp

An unattainable goal. Will o' the wisp is one of the names of the marsh gas phenomenon that causes lights suddenly to appear and disappear over swampy ground (ignis fatuus is another term). Trying to catch a will o' the wisp is impossible, much like trying to catch lightning in a bottle, and so the phrase came to mean anything that can't be done.
See also: will