What you call the end product when you scrounge together any tobacco and weed that you have left sitting in your rolling box, grinder or wherever else you may find some, and put it together to roll a joint. This can only be achieved when your bag of weed has finished at a time when most required. The enjoyment you get from this is unparallel in smoking terms, the name coming from the Mexican Vasectomy firework mentioned by Danny McBride in "Land of the Lost".
Fuck man, the Inbetweeners is on and we've no grass left. See if you can get a Mexican Vasectomy