History: London, England, United Kingdom

From 1899 it was commonly accepted that the name was of Celtic origin and meant place belonging to a man called Londinos; this explanation has since been rejected. Richard Coates put forward an explanation in 1998 that it is derived from the pre-Celtic Old European (p)lowonida, meaning 'river too wide to ford', and suggested that this was a name given to the part of the River Thames which flows through London; from this, the settlement gained the Celtic form of its name, Lowonidonjon. Until 1889 the name officially only applied to the City of London but since then it has also referred to the County of London and now Greater London.
The Welsh word for London is Llundain from Middle Welsh Llundein and the Welsh u descends from either proto-Celtic oi or ou. As with the Romano-British theonym Nodons from Noudont- ", Celtic ou was sometimes transcribed as Latin o. According to established Celtic sound change laws, if Llundain were descended from a former Londiniom, we would expect to see Llondyn or Llyndyn in Welsh, but a hypothetical Lou-nd-e-njo-m would yield Welsh Llundain according to the same sound change principles. The element lou- is a widely attested Proto-Indo-European root meaning ‘wash’.