One way to hold this word: every door turned at once, and then stillness. Somebody decides there is a danger, the locks go on, and for a while nobody crosses the line — no visits, no deliveries, no going out for lunch. What changes between the uses is only how big the locked box is: one cell, one building, or one whole city. The word itself always means the same thing, which is that ordinary movement has stopped and someone else is deciding when it starts again.
lockdown 的核心畫面可以想成：所有的門在同一時間被轉上，然後一片安靜。有人判斷出現了危險，鎖就落下，接下來一段時間沒有人越過那條線——不能會客、不能送貨、不能出去吃午餐。三個用法之間變的只有一件事：被鎖起來的那個盒子有多大，一間房、一棟樓，還是一整座城市。字本身的意思一直沒變：平常的走動停掉了，而且什麼時候恢復由別人決定。
From the word’s history
Deverbal from the phrasal verb lock down — the noun is the action itself, turned into a name for the period it lasts.
lockdown 來自動詞片語 lock down（把東西鎖住、固定住），整個動作被拿來當名詞用。所以它命名的不是一道鎖，而是「鎖著的那一段時間」。
Wiktionary, "lockdown"
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