Picture a small pale light drifting over wet ground at night. You walk toward it; it is still the same distance away. You walk further; the ground is softer now, and the light has moved again. That one scene holds the whole word. Whether it is gas over a marsh or a goal you have been chasing for years, the shape is the same: something visible in front of you that you can follow forever and never stand next to.
From the word’s history
The source entry gives Will (a man's name) + of + the + wisp (a bundle of straw used as a torch) — the light was named as if a man called Will were carrying it.
來源資料把它拆成 Will（男生的名字）＋ of ＋ the ＋ wisp（當火把用的乾草把）。也就是說，古人是把那團光當成「有個叫 Will 的人提著草火把」。這個構詞本身就說明了為什麼它的比喻義這麼自然：它從一開始就是一個會走、會引路的東西，而不是一個死的光點。
Wiktionary, "will o' the wisp"
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