Dindaethwy is a historic commote

Commote

A commote , sometimes spelt in older documents as cymwd, was a secular division of land in Medieval Wales. The word derives from the prefix cym- and the noun bod...

 (or subdivision of a cantref) in north-east Anglesey. It includes Beaumaris and Menai Bridge

Menai Bridge

Menai Bridge is a small town and community on the Isle of Anglesey in north Wales. It overlooks the Menai Strait and lies by the Menai Suspension Bridge, built in 1826 by Thomas Telford...

 (Porthaethwy). The corresponding rural deanery is called Tyndaethwy. The former Aethwy Rural District

Aethwy Rural District

Aethwy was a rural district in the administrative county of Anglesey, Wales, from 1894 to 1974.It was created under the Local Government Act 1894 from the part of the Bangor rural sanitary district which was on Anglesey....

 occupied the predominant non-urban area.

The first element of the name may involve a reference to the hillfort, Bwrdd Arthur, in the (former) parish of Llanfihangel Din Sylwy.