This paper provides an attempt to find the etymology of the Basque case ending "-(r)antz" ‘towards'. I propose that this ending may be a calque of the Latin construction "ad...versum", together with a process of grammaticalization. Firstly, I propose a protoform that could be the origin of all the variants of the Basque suffix. Secondly, it is shown that Basque presents other instances of similar grammaticalization in its case system. Then, I propose the etymology, and provide some parallel examples from Romanic and other Indo-European languages. Finally, some problems and lacunae are mentioned.