The story
From Serbo-Croatian Босна. The river is possibly mentioned for the first time during the 1st century AD by Roman historian Marcus Velleius Paterculus under the name Bathinus flumen. Another basic source that is associated with the hydronym Bathinus is the Salonitan inscription of the governor of Dalmatia, Publius Cornelius Dolabella, where it is said that the Bathinum river divides the Breuci from the Osseriates. Another name could also have been Basante. According to philologist Anton Mayer, the name Bosna could be derived from Illyrian Bass-an-as(-ā), which would be a diversion of the Proto-Indo-European root *bʰegʷ, meaning "the running water."
Etymology from Wiktionary, CC BY-SA 4.0.