Gig Knee is the pain experienced when watching a band play a ridiculously long set. Usually after the 45 Minute mark you get an awful zing in the back of your knees. This rarely disappears until you get the last bus home, Hence the reason for a quick exit at the venue.
Two Friends at a show:

"I've got gig knee!"
"Same here"
"Get off the fucking stage already"
by Andrew Gilson December 27, 2007
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