Ah. He was one of the signatories of the Constitution, known for his particularly large and flamboyant signature, despite not having ended up being particularly famous. So a "John Hancock" is a signature. It's not slang exactly - it's one of those things that no one actually says, but everybody knows what it means. So it was supposed to be something Fraser might think of as sounding dirty and being very American, but it's really a near miss because it has these very unsexy Founding Fathers associations.