One way to hold this word: draw the chart of a government department. At the top there is one box, and the person in it is called the secretary — in a government that title means the head of the whole department, not an office worker. `Under-` simply moves you one box down. That is the entire word: the rank directly beneath the top. It says nothing about what the person does all day, which is why the same title can belong to a politician in one country and to the official who runs the building in another. Find the top box first, and the word tells you where to stand.
From the word’s history
Wiktionary gives the word simply as under- + secretary. Nothing is hidden inside it: the prefix marks rank and the noun names the post it is measured against, which is why every meaning has to be read against whoever the local secretary is.
Wiktionary 直接記載這個字就是 under- 加 secretary。裡面沒有藏東西：字首標的是位階，後面的名詞標的是用來對照的那個職位。所以每一個用法都得先看當地的 secretary 是誰，才讀得出這個人有多大。
Wiktionary, "undersecretary"
登入就可以看每一支怎麼連回核心畫面。