There are several possibilities as to the origin and, therefore, actual meaning behind this cryptic phrase. One, that appears to be reasonable is "... Hell for leather is a statement that is often confused with "Hell bent for leather". Hell for leather, in American vernacular, refers to an arduous walk that may have been strewn with difficulties and was a strain on footwear. A long and difficult walk, such as over rough terrain, might be referred to as hell for leather because of the abuse the leather footwear sustained during the walk. "Hell bent for leather" has many uses and the most popular american use goes back to the 19th century american west when a particular livestock animal, such as a cow, bull or horse would be particularly difficult to handle. One of these troublesome creatures would cause their handler so much trouble that the owner or handler considered slaughter of the animal and turning the carcass into leather. When a horse or cattle became difficult to handle they were called "Hell bent for leather" meaning that the animal was hell bent to become a leather good." (Source: The Phrase Finder).
Another, with good sources states: "Robert L. Chapman's "New Dictionary of American Slang" (Harper &
Row, 1987, ISBN 0-06-181157-2) says: "hell-for-leather or hell-bent-for-leather adv "from late 1800s British" Rapidly and energetically; =all out, flat out. "You're heading hell-for-leather to a crack-up" [origin unknown; perhaps related to British dialect phrases "go hell for ladder, hell falladerly, hell faleero", and remaining mysterious even if so, although the "leather" would then be a very probable case of folk etymology with a vague sense of the "leather" involved in horse trappings."
So... take your pick...