(1) A terse and deliberately understated manner of expressing that something is less than hoped. (2) a way of understating a substantial problem, mess, or catastrophe.
Sometimes expressed with a suspenseful interlocutor such as "ummm," "errr," or a gesture of consideration like a glance or a finger on the lip.
Q: How was your first day at the marketing firm with your new boss?
A: I just got home from work after a day with my crazy boss, umm, not good.
Ex 2: Did you see that plane hit the tower? Wow, not good.