"If you come out of a concert and the next morning you don't have one of these and/or severe bruising from the mosh pit, you might as well have not bothered." - Hazelnut, just now, on how to tell whether a gig was win or fail.
At this moment, you are asking me what exactly a bangover is. Very well then. I will tell you. A bangover is the pain your get in your neck and upper back from too much headbanging the night before. Often it is accompanied by shoulder ache from prolonged fist-pumping and an actual real life hangover.
You may now be asking me, what is the point of doing all this if you just feel grotty the next morning. I will tell you. It's kind of a badge of honour to have a bangover, it shows you had a good time and put some effort in. Metalheads are big on putting effort in.
As a term though, Bangover seems to have first appeared on Municipal Waste's 2005 album "Hazardous Mutation" as a song name. It's about having one. Bangovers have been about themselves for a ridiculously long time, though.
They are strangely satisfying, though.