Etymology

From a previous undeclinable Eastern Baltic *dewin-, from Proto-Balto-Slavic *newin-, from Proto-Indo-European *h₁néwn̥, *néwn̥ ‎(“nine”). Probably from the same stem as *néwos- ‎(“new”); apparently Proto-Indo-European had a base 4 numeric system, so that, after two 4's (= 8), 9 was the first (“new”) to be part of a complex numeral (compare Ossetian фараст ‎(farast, “nine”) = фар ‎(far, “over”) + аст ‎(ast, “eight”)).

The initial d in Eastern Baltic and Slavic is usually explained as dissimilation, given the two n's in *newin-, probably also under the influence of the initial d in desmit. A more recent suggestion is that Proto-Indo-European *néwn̥ < *h₁néwn̥, in which the h₁n sequence would yield an articulation similar to a d. This would have led to dialectal variation (*néwn, *déwn), with both forms preserved in parallel, the former giving rise to the Eastern Baltic terms, the latter to their Old Prussian counterpart. Cognates include Latvian deviņi, Old Prussian newints ‎(“ninth”), Old Church Slavonic дєвѧть ‎(devętĭ), Russian, Ukrainian де́вять ‎(dévjatʹ), Belarusian дзе́вяць ‎(dzjévjacʹ), Bulgarian де́вет ‎(dévet), Czech devět, Polish dziewięć, Gothic, Old High German 𐌽𐌹𐌿𐌽 ‎(niun), German neun, English nine, Sanskrit नवन् ‎(návan), Ancient Greek ἐννέα ‎(ennéa) (< *en néwa), Latin novem, Tocharian A, Tocharian B ñu.