This paper discusses the etymology of two lexical items of Terena, an Arawakan language from southwestern Brazil. Etymologization of the nouns mojénoti ‘orphan’ and moʃêu ‘barren, sterile’ shows that they preserve traces of the Proto-Arawakan Privative prefix *ma-, now demorphologized as part of synchronically unanalyzable roots. Since PA *ma- appears as mo- in Pre-Terena, Proto-Mojeño and Baure, the present work also provides evidence for the shared development *a > o, of still unclear conditioning, for the Bolívia-Paraná subgroup of the Arawakan language family, improving on an earlier formulation by Payne (1991). Other developments directly related to the comparative equations supporting each etymology are also discussed. A direct implication of these findings is that the hypothesis that PA *ma- > o- in Terena, advanced by Danielsen, Dunn & Muysken (2011), can be safely rejected.