Think of a thin, invisible wall sitting on the outside of something. A mosquito flies at it, a raindrop lands on it, an idea arrives at it — and none of them get in. They stop and turn away. Nothing is destroyed; the only thing that happens is distance. English builds the word exactly that way: repel means push back, and -ent names the thing doing the pushing.
repellent 可以想成一層看不見的薄膜，貼在人或東西的外面。蚊子飛過來、雨滴落下來、一個念頭冒出來，碰到這層膜都進不去，只好退開。重點不是把對方消滅，而是拉開距離。這個畫面同時解釋了噴在皮膚上的防蚊液、噴在鞋面上的防水劑、會撥水的布料，還有讓你一看就想退一步的那種厭惡。
From the word’s history
From Latin repellens, from repel plus the ending -ent; equivalent in modern English to repel + -ent, literally the thing that drives back.
來自拉丁文 repellēns，在現代英文裡等於 repel（推回去）加上 -ent（做這件事的東西或人）。所以字面上就是「把東西推回去的那個」，這正好是今天四個用法共同的畫面。
Wiktionary, "repellent"
登入就可以看每一支怎麼連回核心畫面。