The Welsh name for England is Lloegr (/ɬɔigr/). The etymology of this name is a mystery. According to this site, it first appeared as Lloegyr in an early 10th century prophetic poem called Armes Prydain. A variant of the name, Lloegrwys, or “men of Lloegr”, was in use before then and more common. In early poetry, the names used for the English included Eingl (Angles) and Iwys (Wessex-men) – they are called Saeson (Saxons) in modern Welsh. Some scholars believe that Lloegr originally referred to the kingdom of Mercia, and eventually came to mean the whole of England.
There’s a thread on this forum in which a number of possible etymologies of Lloegr are discussed. Here are some of them:
- it comes from the Middle English name for England, Loegres
- it comes from Legorencis Civitas, the Roman name for Leicester, which was probably derived from a local Celtic name
- it means the “lost land(s)”
- it’s named after Locrinus, the son of Brutus (from Geoffrey of Monmouth)
More details: http://www.old-north.co.uk/Holding/celt_lloegr.html