This is City shorthand for the Sterling against the US Dollar (£/$). So called because of a cable that was laid across the Atlantic linking New York and London in 1858. The first message was a congratulatory one sent by Queen Victoria of Great Britain to President James Buchanan. The telegraph wire broke after 3 weeks and wasn't properly repaired until 1866.
This trans-Atlantic cable was then used to transmit, amongst other things, the pound sterling rate versus the dollar (£/$) and thus the nickname "cable" was born.
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