An unflappable person stays calm when things go wrong. Not calm because nothing is happening — calm while the plane is shaking, while the customer is shouting, while the numbers on the screen are all wrong. That is the whole point of the word: it only means something when there is real trouble around. People use it about pilots, nurses, teachers of small children, goalkeepers, news readers, and anyone whose job is to be the steady one in a bad moment. It is a compliment, and it is friendly and a bit informal — you would say it about your boss to a colleague, and you would see it in a newspaper, but it would sound too light for a very serious formal report. One honest thing worth telling students: it describes how someone looks and behaves from the outside, not how they feel inside. An unflappable nurse may be frightened; you just cannot tell. So the word is really about a surface that does not crack. There is also a small warm edge to it. Because it is a slightly playful-sounding word — flap, flappable — it often carries a hint of admiration mixed with amusement, as if the speaker is a little impressed that anyone could stay that calm.

處變不驚的、泰然自若的（再亂也不慌）；口語一點可以說「超穩的」、「不動如山」。