"拔腿就往外跑" 的英文表达可以是 "run away at the drop of a hat" 或 "take to one's heels"。

"Run away at the drop of a hat" 这个短语的来源可能源于戏剧表演中常见的情节，即演员被吓到时会立刻逃跑。"Drop of a hat" 在这里指的是一个帽沿，意思是毫不考虑地、毫无顾忌地。因此，"run away at the drop of a hat" 的意思就是在第一个机会就立刻逃跑。

例句：

He's always ready to run away at the drop of a hat if he thinks he's in trouble. 他一想到有麻烦就立刻逃跑。
She's so scared of spiders that she'll run away at the drop of a hat if she sees one. 她非常害怕蜘蛛，一看到就立刻逃跑。

"Take to one's heels" 这个短语也表示立刻逃跑的意思，其中的 "heels" 指的是脚后跟。这个短语可能源于某些动物在受到威胁时会立刻逃跑，尤其是用脚后跟猛冲的动物。

例句：

When the dog started barking, the thief took to his heels and ran away. 看到狗在叫，小偷立刻逃跑。
As soon as the alarm went off, the burglars took to their heels and fled the scene. 一声警报响起，窃贼立刻逃跑。