The act of clenching your jaw muscles toghether and grinding your teeth together whilst under the influence of either ecstasy or speed. Often alleviated by chewing gum; or more commonly these days, baby dummies. Magnesium can often help to looses the jaw bones and prevent teeth-grinding. In exceptional cases; like an xtc overdose, it is possible to have complete jawlock; where you physically cannot open your mouth (though this is rare).
'Man, I'm gurning tonnes tonight'
'Look at that bloke gurn'