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The name Dubris, Dover's old name, comes from Celtic, Dubras, and means: The Waters. The name Dubris, with an i, is probably a dative-locative form giving the meaning: by the waters. It is a river name, no-doubt refering to our river Dour, but which remained the name of the town.
The name Kent is also Celtic, from Cantion, with the probable meaning of: coastal district, or even corner land, or land on the edge. It could also mean: land of the armies.