In the not too distant past the prices quoted were obtained off the one and only transatlantic telephone/telegraph link (landed in Cornwall). Hence 'Get a price off the cable for GBP/USD' became abbreviated to 'cable'. The dealer would call out 'cable' and the telegraph operator would know what was meant.
Trader jargon referring to the Sterling/US Dollar exchange rate. So called because the rate was originally transmitted via a transatlantic cable beginning in the mid 1800's.
