Chiefly Southern U.S.
Probably alteration of scrat, from Middle English, hermaphrodite goblin, from Old Norse skratte, wizard, goblin
Old Scratch, like Old Nick, is a nickname for the devil. In the last century it was widely used in the eastern United States, especially in New England, as is evident from the Devil's name for himself in the Stephen Vincent Benét short story "The Devil and Daniel Webster." Now the term has been regionalized to the South. Old Scratch is attested in the Oxford English Dictionary from the 18th century onward in Great Britain as a colloquialism: "He'd have pitched me to Old Scratch" (Anthony Trollope, 1858). The source of the name is probably the Old Norse word skratte, meaning "a wizard, goblin, monster, or devil."