...can happen as a result of carrying Mexican i.e. sans pistol holster/proper retention device--gun usually tucked into waistband.
Literally: to accidentally blast one's own genitalia off via a negligent discharge of a firearm that is being "carried Mexican."
Mexican castration almost happened to Plaxico Burress when his (.40 S&W) Glock 23 went off and struck him in the thigh at that New York night club because he had his piece tucked into the waistband of his sweatpants. But seriously, despite the fact that he could have shot his own balls off, who wears sweatpants to a nightclub?