Newcassle is the spelling of the Geordie pronunciation of the place name Newcastle, geordie's choosing to completely ignore the T in the place name. Recently, this pronunciation of Newcastle has been adopted by those outside the city, noteably Match of the Day pundits such as Alan Hansen and Mark Lawrenson. These people are known as Tw@'s!
Steve: So hank, where you from?
Hank: I'm from Newcassle!
James: Don't you mean Newcastle?
Hank: No way, man, I'm from fuckin' Newcassle!