Gurn
The act of ones jaw vibrating, clenching or protruding. This happens whilst under the influence of various drugs such as MDMA, Ecstasy, Speed etc. The morning after gurning all evening is very painful, and this is how people ruin their teeth on drugs.

Gurn
to gurn;gurning is when you have took ecstasy, cocaine or amphetamin which has resulted in your jaw moving from side to side 'rocking'. this is when you are gurning.
"wow, im gurning off that e' mate"
"check that gurner over there"
"your going to gurn in a minute"