The reason that bootlegging is called bootlegging is because of the way that people would carry illegal alcohol in their boot leg during transport to get past authorities.
They would hide the illegal alcohol in the leg of their boot and would cover it with their pants so that nobody would see the illegal alcohol and they could get from one place to another and then either sell the illegal alcohol or drink it.
Bootlegging is basically the illegal sell of alcohol but can also be the illegal sell of any other goods.
I've also heard people call doing anything illegal bootlegging.
Basically the term bootlegging derived from the practice of American frontiersmen who carried bottles of illicit liquor in the tops of their boots.
In its original sense, bootlegging blossomed during the Prohibition era in the USA (1920–33), and helped create powerful gang bosses.