멘붕 ~ mental breakdown (literally “mental collapse”)
This is another colloquial word that is not in the dictionary and comes from two words:
멘탈 ~ mental
붕괴 ~ collapse
Examples of when it is used:
Most of you have probably seen the desperate kind of messages people will post on facebook before exams. They fall into the category 멘붕.
In a SNSD song, one of the girls sings about having a 멘붕 because her boyfriend wants to see her without make-up (…well… Anyway).