Auch is a very ancient town. The name of Auch comes from the Aquitanian tribe that inhabited the area at the time of the Roman conquest in the 50s BC. The name of this tribe, as recorded by the Romans, was Ausci (pronounced in Latin), singular Auscus. Aquitanians spoke a language related to the old Basque language, and a striking fact is that the name Ausci seems related to the native name of the modern Basques, who call themselves Euskal (pronounced in Basque).
At the time of the Roman conquest, the native name of Auch, as recorded by the Romans, was Elimberris. Elimberris is itself a variant of Iliberri, where Ili- comes from the Iberian word iltir' or iltur (the "t" is mute) meaning town, city, oppidum, a frequent prefix in names of Iberian cities, and which was probably borrowed into Basque and also Aquitanian (cf. modern Basque hiri, meaning "town"). In modern Basque there is also the word berri which means "new", so Iliberri is often interpreted as meaning "new oppidum" (based on the fact that Aquitanian and old Basque are related). However, this interpretation should be taken with caution. In the south of Spain, near Granada, an area far away from the Basque homeland, there was an Iberian city whose name the Romans also recorded as Iliberi, and here it seems the name recorded by the Romans is the phonetic transcription of Iberian Ilbirir, which comes from the Iberian stem ilbi- (meaning unknown), and not at all from an hypothetical il(t)i- (town) and Basque berri.
The Romans renamed the town Augusta Auscorum (or Augusta Ausciorum), which means "Augusta of the Ausci". Eventually, Augusta was dropped, and the name evolved into modern Gascon Aush, and modern French Auch. Inhabitants of Auch are known in French as Auscitains (pronounced )
Capital of Gascony
In 409, when Eauze, the capital of Novempopulana, was ruined by the Vandals, Auch replaced it as the capital of Gascony. It became the seat of a Catholic archdiocese, the Archdiocese of Auch, covering the whole of Gascony, which lasted until the French Revolution, and whose archbishops claimed the title of Primate of Aquitaine, Novempopulana, and of the kingdom of Navarre.