Cwmyoy is an extensive rural parish in Monmouthshire, Wales (Welsh: Cwm Iou) for the valley and parish, (Welsh: Cwm-iou) for the village. The standard Welsh name is Cwm Iau / Cwm-iau. In the Gwentian dialect of Welsh that was spoken here until the late 1800s, the name was pronounced as Cwm Iou ('ou', also spelt informally 'oi', for standard 'au' is a common feature of south Wales Welsh). 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.