"Head Bent for Leather" is probably a mondegreen of the phrase "Hell-bent for leather". Hell bent for leather seems to be a very recent usage in which two phrases: hell-bent and hell for leather have been run together. The bent in hell-bent means "determined" or "resolute", as in "bent on revenge", so hell-bent means "intent on going to hell". Hell for leather, on the other hand, means "fast". It occurs twice (1889, 1893) in Kipling's stories of the British Army in India. In both cases it refers to horse-riding and leather probably refers to the saddle. It may have originated as Army slang or it could possibly have been one of Kipling's inventions. For the meaning, we can only suggest that you ask the next person who uses it in conversation with you. It is understood to mean "as fast as possible", having retained the meaning of hell for leather with the extraneous bent. Source: http://www.takeourword.com/TOW154/page2.html
