He is a septic who moved here from New York.
septic
idiom Slang
Cockney rhyming slang · East London
Yank
Origin
via septic tank
The locals at the pub always joke with him because he is a septic, but they really like his accent.
While the term might sound strange to visitors, it is common rhyming slang for a septic tank, which people use to describe an American traveler.