Re: "get a guernsey"?
"But I don't know why we call it a guernsey"
It was originally a knitted product from the Channel Island of Guernsey. My grandfather born in the late 19th century used the term and it was considered part of the Potteries dialect of Biddulph Moor. As someone else has pointed out the word was also corrupted into "ganzie".
A more common name for that item of clothing in England is a jersey - from another Channel Island, Jersey.
Jersey wool, now supplanted by other fibres, produced a fine stretchy material ideal for things like shirts etc.