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).
Eelco is the name given to a stoner, in other words one who frequently participates in drug abuse as a recreational activity. It can also be used as a slur for those who are known to curb crawl in the late hours picking up prostitutes in Amsterdam