Will o' the wisp

What does the phrase "will o' the wisp" mean, and where does it come from? Thanks.

From the Oxford English Dictionary:

[Etymology:] "Will-o-the-wisp . . . [orig. 'Will with the wisp': see WILL sb.3 and WISP sb. Cf. JACK-O'LANTERN, and, for the second element, G. 'irrwisch'.]"

"Will, sb.3 Abbreviated pet-form of the Christian name 'William'."

[Definitions for WISP include a handful of hay, a bunch of straw used as a torch, and the like, and then this:] "A marsh-fire, WILL-O'-THE-WISP; also the light supposed to be carried by the sprite."

[Meaning:] " = IGNIS FATUUS; fig. a thing (rarely a person) that deludes or misleads by means of fugitive appearances."

Si it's the "light supposed to be carried by the sprite --- a thing that deludes or misleads by means of fugitive appearances" ???? Something that may or may not exist, seen intermittantly, a possibly mistaken perception?

So it's the