Because it had water, it became known as the water closet. Somehow the name stuck, and plumbers continue to refer to a toilet as a water closet. That’s what the “WC” imprinted on most toilets stands ...
If you’re cutting a curve, Martin recommends forming your line using pex, hose or fiberglass tent poles ... a pro if you have to do major cuts or are unsure about crossing over plumbing and electrical ...