Massad Ayoob did an ariticle on Mexican Carry a few years back. While the practice obviously did not originate in Mexico, the term apparently got coined after the Mexican revolution of the early 20th Century, when Mexicans lost their right to be an armed society. Packing a gun in a rig was replaced by stuffing the muzzle inside the waistband for those who were defiant and had no intentions of running around disarmed. In this day and age of permitted concealed carry and better concealment leather than ever, it is not a good practice.