The term originated around the 1920s, though the practice probably predates that. Generally, a bootleg mine (sometimes called a Bootleg Pit) is a small mine dug by a handful of men. They were frequently dug by coal miners off official tunnels in order to procure additional, free coal for themselves, a practice that causes additional ramifications when fighting mine fires. Sometimes small pits are hidden under houses or outbuildings. Usually, the mine is not large enough to turn around in. The pits are known for being unsafe, and often causing collapse.
The practice has died away in the United States; an American with simple equipment cannot dig enough coal in a day to reach a living wage. Bootleg mines in China are very common, as are the fatalities resulting from unregulated mining.