機器上的洩壓口
看得到、摸得到的那一個：壓力鍋蓋上的小金屬帽、鍋爐上的閥。
noun · B2
A part on a machine such as a boiler or a pressure cooker that opens by itself and lets gas or steam out when the pressure inside gets too high, so the container does not burst.
安全閥、洩壓閥
核心畫面
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…更多收起
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 entry for “safety valve”, etymology 1 (kaikki extract supplied in the evidence packet)
Two branches
情緒的洩壓口
把同一個順序搬到人身上：情緒也會一直往上累積，也需要一個事先留好的出口。
noun · B2
Something that lets strong feelings out in a way that harms nobody, so that the feelings do not build up inside a person or a group.
情緒的出口