看不見的身影
這一支就是核心畫面本身：有東西在那裡，但你抓不到、也證明不了。
noun · B2
A ghost: the spirit of a dead person that some people believe can appear and be seen. It is a light, everyday word, used in ghost stories and at Halloween more than in serious writing.
鬼 / 幽靈
核心畫面
One way to hold this word: a spook is something that appears for a second at the edge of your eye and is gone before you can look at it properly. Everything else in the word can be traced back to that flash. The jump it gives you is the verb. The person who moves through a room the same way, seen by nobody, is the spy.
可以把 spook 想成眼角閃過去的一個影子：你轉頭，它已經不在了。這個畫面裡有兩樣東西——一個看不見的身影，和它在…更多收起
可以把 spook 想成眼角閃過去的一個影子：你轉頭，它已經不在了。這個畫面裡有兩樣東西——一個看不見的身影，和它在你身上留下的那一下心跳。字的各種意思，就像從這個畫面的兩邊各長出一條線：一邊是「不現身的東西或人」，一邊是「被嚇到的那一瞬間」。From the word’s history
English borrowed spook from Dutch spook, meaning ghost, which goes back to Middle Dutch spooc; German Spuk and Swedish spoke belong to the same family. So the ghost is not a metaphor here - it is the meaning the word arrived with, and the frightening and the spying grew from it inside English.
英文的 spook 借自荷蘭文 spook（鬼），再往前是中古荷蘭文 spooc；德文 Spuk、瑞典文 spöke 都是同一家人。所以「鬼」不是後來的比喻，而是這個字進英文時帶著的原意。「嚇人」和「間諜」是它進了英文以後才長出來的。
Wiktionary (kaikki extract) entry 'spook': Borrowed from Dutch spook (ghost), from Middle Dutch spooc; cognate with German Spuk, Swedish spöke. source_ref kaikki:3f6f69a190036ae4:782660674:19630
Two branches
被嚇的那一下
同一個畫面換個角度看：不是看那個影子，而是看影子在人或動物身上造成的反應。
verb · B2
To frighten a person or an animal suddenly, so that they jump, freeze, or run away. It can also mean to make people suddenly nervous about a situation, so that they pull back from it.
嚇到 / 使受驚