A Knuckle-buster is an old-fashioned term for a manual credit card machine. These machines were originally used to process paper credit card slips. The name "Knuckle-buster" comes from the fact that the machine would often jam, and the only way to fix it was to use a lot of force - hence, busting your knuckles! Nowadays, of course, credit card processing is done electronically, so you don't have to worry about busting your knuckles anymore.
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