まずい is used for describing the horrible flavor of something, but is also used as an expression by itself.

まずい is used to comment on something you see coming that you are dreading.

You went drinking instead of studying last night and your teacher asks everyone to close their books and take out paper. まずい！

The kids you are watching just got into chocolate covered coffee beans. まずい！

*This was just a little language point I was taught stemming from my vocabulary list.  I will likely alternate weeks between something like this and more vocabulary.