Fylleangst means the feeling of fear of what you could have done in a preceding blackout caused by alcohol.
A common and unique word in Norway, not known to exist in any other language than Norwegian. This uniqueness is probably caused by the Norwegian drinking culture, socially awarding the one who gets most drunk, and showing disgust against modest drinkers.
I woke up with the worst fylleangst this morning. As I was about to call the friends I remember I was with last night to find out what I did after the blackout kicked in, I could see I had a text message that read "hey!what's up with greasing your balls in with peanut butter, and letting MY dog lick them clean?!?if you're wondering what I, or what the rest of Mankind in general think of it: SOOO WRONG!!"