From The Jargon File (version 4.4.7, 29 Dec 2003):
scratch
1. [from scratchpad] adj. Describes a data structure or recording medium
attached to a machine for testing or temporary-use purposes; one that can
be scribbled on without loss. Usually in the combining forms scratch
memory, scratch register, scratch disk, scratch tape, scratch volume. See
also scratch monkey.
2. [primarily IBM, also Commodore] vt. To delete (as in a file).