Cwmyoy is an extensive rural parish in Monmouthshire, Wales. The standard Welsh name is Cwm Iau or Cwm-iau. In the Gwentian dialect of Welsh that was spoken here until the late 1800s, the name was pronounced as Cwm Iou. The 'English' name is in fact this local dialect form in a more English spelling. The name of the valley probably originates from the Welsh word iau meaning yoke, in reference to the shape of the hill surrounding it.