Londinium.

Uncertain, but likely from Proto-Celtic *Londinyom (“place that floods”), from Proto-Indo-European *lendʰ- (“heath; wild land”) + *-injo-, a suffix forming specific nouns and found in other Romano-Celtic placenames. Cognate to Proto-Celtic *landā (“low-lying land”), whence Old Irish land, Welsh llan, as well as *londos (“subduing > fierce”, adj.), whence Old Irish lond. Details An earlier, more difficult proposal derives it from *Φlowonidonyom, morphologically adapted from a pre-Celtic Indo-European substrate word meaning something like “Boat River, Unfordable River” or “Flooding-River” and made up of Proto-Indo-European *plew- (“to flow”) and a disputed *neyd- (“to flow”). Forms in -don- could be artificial Latinizations modelled after e.g. Old English Wreocen < Latin Viroconium (modern Wroxeter). Appears as Middle Welsh Llundein, Old English Lunden, which may not continue the Classical Latin form, but may continue Late British Latin *Lundeinju if the medial -i- of the original word was short. Cf. Etymology of London.