Picture something sealed with the pressure rising inside it, and one small opening set to give way first. It opens before anything else can break, a little is let out, and the danger never arrives. Both uses of the words keep that same order of events: pressure, a planned way out, nothing bursts.
safety valve 可以想成「一個封住的容器，裡面壓力一直往上升，而上面留了一個會先讓步的小開口」。它趕在任何東西壞掉之前先打開，放掉一點，危險就沒有發生。這個詞的兩個意思都照著同一個順序走：壓力累積、有一個事先留好的出口、結果什麼都沒爆開。
From the word’s history
Wiktionary gives the origin simply as safety + valve: the valve that is there for safety.
Wiktionary 只寫了 safety + valve：就是 safety（安全）加 valve（閥門），字面上「為了安全而設的閥」。這個字面意思和它今天的用法幾乎重疊。
Wiktionary, "safety valve" (etymology 1)
登入就可以看每一支怎麼連回核心畫面。