Etymology and names
Before the 5th century, the city was known as Argantorati (in the nominative, Argantorate in the locative), a Celtic Gaulish name Latinized first as Argentorate (with Gaulish locative ending, as appearing on the first Roman milestones in the 1st century CE), and then as Argentoratum (with regular Latin nominative ending, in later Latin texts). That Gaulish name is a compound of -rati, the Gaulish word for fortified enclosures, cognate to the Old Irish ráth (see ringfort), and arganto(n)- (cognate to Latin argentum, which gave modern French argent), the Gaulish word for silver, but also any precious metal, particularly gold, suggesting either a fortified enclosure located by a river gold mining site, or hoarding gold mined in the nearby rivers.
After the 5th century, the city became known by a completely different name Gallicized as Strasbourg (Lower Alsatian: Strossburi, [ˈʃd̥rɔːsb̥uri]; German: Straßburg, [ˈʃtʁaːsbʊɐ̯k]). That name is of Germanic origin and means "Town (at the crossing) of roads". The modern Stras- is cognate to the German Straße and English street, all of which are derived from Latin strata ("paved road"), while -bourg is cognate to the German Burg and English borough, all of which are derived from Proto-Germanic *burgz ("hill fort, fortress").
Gregory of Tours was the first to mention the name change: in the tenth book of his History of the Franks written shortly after 590 he said that Egidius, Bishop of Reims, accused of plotting against King Childebert II of Austrasia in favor of his uncle King Chilperic I of Neustria, was tried by a synod of Austrasian bishops in Metz in November 590, found guilty and removed from the priesthood, then taken "ad Argentoratensem urbem, quam nunc Strateburgum vocant" ("to the city of Argentoratum, which they now call Strateburgus"), where he was exiled.