I know what Yorks, Lancs and Geordie refer to, but what's/where's "Scouse"?

Scouse, of Liverpool, the Liverpudlian dialect
Scouser, a Liverpudlian.

A language all of its own, like Euskara

Edit: All mouth and trousers. General throughout UK I think, though I've no idea where it originated. Some sites suggest the meaning arises from the implication that there is nothing useful under the trousers, any bulge is just the cut of the cloth.
Rod