Danube
"Danube" is an Old European river name derived from a Proto-Indo-European "*dānu". Other river names from the same root include the Dunaj, Dzvina/Daugava, Don, Donets, Dnieper, Dniestr, Dysna, Tana and Tuoni. In Rigvedic Sanskrit, "dānu" means "fluid, drop", in Avestan, the same word means "river". In the Rigveda, "Dānu" once appears as the mother of Vrtra, "a dragon blocking the course of the rivers." The Finnish word for Danube is Tonava, which is most likely derived from the word for the river in Swedish and German, "Donau". Its Sámi name "Deatnu" means "Great River". It is possible that "dānu" in Scythian as in Avestan was a generic word for "river": "Dnieper" and "Dniestr", from "Danapris" and "Danastius", are presumed to continue Scythian "*dānu apara" "far river" and "*dānu nazdya-" "near river", respectively.
List of reconstructed Dacian words
The city and river (a tributary of the Danube) in eastern Romania called Cernavodă. In Slavic, the name means "black water". The same town in Antiquity was known as "Άξίοπα" ("Axiopa") or "Άξιούπολις" ("Axioupolis") and its river as the "Άξιος" ("Axios"). The working assumption is, therefore, that "Axiopa" means "black water" in Dacian. According to the known rules of formation of IE composite words, this breaks down as "axi" = "black" and "opa" or "upa" = "water" in Dacian (the "-polis" element is ignored, as it is a Greek suffix meaning "city"). The assumption is then validated by examining cognate placenames. The "axi" element is validated by another tributary of the Danube called the "Axios", which is today known as "Crna reka" (located in Republic of Macedonia "black river") and by the older Greek name for the Black sea, "Άξεινος πόντος" ("Axeinos pontos", later altered to the euphemism "Euxeinos pontos" = "Hospitable sea"). The "opa/upa" element is validated by the Lithuanian cognate "upė" ("water"). This etymology is questioned by Russu: "Axiopa", a name attested only in Procopius' "De Aedificiis", may be a corrupt form of Axiopolis. Even if correct, however, Russu's objection does not invalidate the decipherment of the "axi-" element.