真的那座會拉起來的橋
這就是畫面本身，還沒有被借去比喻。
noun · B1
A bridge that can be pulled up. It is built at the way into a castle, so that the people inside can stop anyone crossing; or it is built over water, so that it can be lifted to let a tall boat pass under.
吊橋
核心畫面
One slab of bridge on a hinge, with ropes running back inside the wall. While it lies down it is a road, and people walk in. The moment it is pulled up, the road is simply gone and the wall is solid again. Opening and closing are the same movement, and the hand on the rope is always on the inside.
可以想成一塊裝了軸的橋面，繩子從牆裡拉出來。橋放下來的時候，它是一條路，人走得進來。橋拉起來的時候，路就不見了，入口…更多收起
可以想成一塊裝了軸的橋面，繩子從牆裡拉出來。橋放下來的時候，它是一條路，人走得進來。橋拉起來的時候，路就不見了，入口變回一道牆。這個畫面有兩個地方值得記住。第一，開和關其實是同一個動作，差別只在方向。第二，拉繩子的手一直在牆的裡面——決定要不要開門的，永遠是已經在裡面的人。drawbridge 之後長出來的幾個比喻用法，都是從這兩點來的。From the word’s history
Middle English drawebrigge; by surface analysis, draw + bridge — a bridge that is pulled.
詞源把這個畫面直接寫在名字裡：drawbridge 來自中古英語 drawebrigge，拆開就是 draw（拉）加 bridge（橋）。也就是說，它不是靠位置定義的橋，而是靠動作定義的橋——有人拉，它才動。這也解釋了為什麼後來的比喻，講的都不是橋本身，而是「拉起來」這個動作。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:432543105:15550): 'Inherited from Middle English drawebrigge; by surface analysis, draw + bridge.'
Two branches
把橋收起來：切斷通路
這一組全部來自「拉起來」那一下。真實的橋收起來，人就進不來；這個動作被借去講人和團體，於是有了「我不參與了」和「不再讓人進來」兩種意思。
noun · C1
If you pull up the drawbridge, you stop taking part in something and keep yourself apart from it, leaving other people to carry on with it without you.
抽身不管