Emergency Exit
noun
You know how 非常 means emergency and how 口 can be some kind of doorway (like in 入り口, for example)? Well, if that's the case, then you have an emergency doorway. Usually you run away from emergencies, though, making this an emergency exit.
The reading is the words 非常 and 口 combined together. The thing that makes this weird is that it's a mix of the kun and on'yomi readings. I guess things get all jostled up when you have yourself an emergency.
この建物には非常口が二ヶ所あります。
There are two emergency exits in this building.
非常口はそこの通路を左に行ったところにあります。
You will see the emergency exit when you turn left at that corridor.
不良高校生がクラスをサボってくつろぐ場所と言えば非常口のかいだんの下ですよね？
When you think of a place where bad high schoolers skip class and hang out, it would be under the emergency exit staircase, right?