What is Mcgarett?
1.
Fifty pounds (ticket-tout slang, UK)
“There’s a McGarett in it for your friend. And also one for you, of course,” Alfie said. “Think about it.”
In the ticket-tout slang that Alfie had picked up from George, a McGarrett was fifty pounds -- cash in hand, of course. (from Mind's Eye by Paul McAuley)
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