One picture covers every use: something already standing in the way before the trouble arrives. On an ordinary day it looks useless — the antibiotic taken the morning before the tooth comes out, the mask in the drawer, the rule nobody has had to apply, the backup that runs at eleven and reports nothing. You only find out what it was for on the day the trouble shows up and does not get through. Everything this word does is a version of that: it is never about fixing, always about being early.
From the word’s history
Wiktionary gives prophylactic as from Latin prophylacticus, from Ancient Greek prophulaktikós. The packet supplies the route only and no meaning for the Greek parts, so the guard-standing-in-front picture above is offered as a teaching image, not as a claim about the root.
Wiktionary 記載 prophylactic 來自拉丁文 prophylacticus，再往前是古希臘文 προφυλακτικός。手邊的資料只給了這條來路，沒有給希臘字各部分的意思。所以上面那個「先站在前面擋著」的畫面是教學用的想像，不是對字根的主張。
Wiktionary, "prophylactic"
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