The painful aching of the neck muscles after moshing, jumping around, headbanging and generally acting stupid at a metal/punk show.
Symptoms are a dull ache, head flopping forwards, and difficulty moving the head from side to side.
Can last anything from a day to a week.
Greg: Dude, what's up with you? Why can't you move your neck?
Jeb: Man, I did some serious...aahhhh...moshing at that show last night. I've got a bad......argh....case of mosher's neck.
Greg: Oh, right.