Picture the line that is left when something is pushed through a surface: a plough through soil, a wheel through mud, water down a . The line is a small valley. The middle is pressed down and the material that was there is up along both sides. It is long, and it is deep enough to lay a finger in. Once you can see that line, the face works the same way. Worry or hard thinking pulls the skin of a forehead into exactly the same shape: a long fold pressed down, with the skin standing slightly higher on each side. Field or forehead, the word is asking you to see one deep line pressed into a surface.
From the word’s history
The Wiktionary entry in the packet traces furrow back through Old English furh to an old root meaning to dig. That fits the picture the word still carries: a furrow is something that has been dug or pushed open, never something drawn on top of a surface.
資料包裡的 Wiktionary 條目把 furrow 上溯到古英語 furh，再往上追到一個意思是「挖」的古老字根。這正好對上這個字今天的畫面：furrow 一定是被挖開、推開來的，不是畫在表面上的線。
Wiktionary (kaikki extract in evidence packet), source_ref kaikki:3f6f69a190036ae4:955993085 — Middle English furgh, Old English furh, Proto-Germanic *furhs, PIE *perḱ- (to dig).
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