 The name probably means "the strong or swift river".This is on the assumption that the Brittonic name of the river was "Isurā", because the Roman name for Aldborough was "Isurium"; intervocalic "s" is known to have been lost in Brittonic at an early date.This explanation connects the river name with an Indo-European root "is-" meaning "strong" and the names of the Isar in Germany and the Isère in France.