This paper presents an etymological investigation of the Terena noun for 'mouth',-pâho, which, according to current comparative Arawakan linguistics, lacks known cognates. I show that cognates of this Terena noun exist in Mojeño, a close relative of Terena within the Bolivia-Paraná subgroup of the Arawakan family. In Ignaciano and Trinitario, the best known modern varieties of Mojeño, the cognates of-pâho are semantically-shifted nouns meaning 'door'. I propose an account of the semantic and formal relations between these forms via an etymological source *-paho 'mouth' and a compound noun *paho-peti 'door' (lit. 'mouth (of the) house'). This account relies on a more general pattern of noun formation in the Arawakan family, regular phonological correspondences and is consistent with modern views on the nature of diachronic metaphorical extensions in lexical semantic shift. The evidence presented and argumentation built to support this analysis adds Terena evidence to the correspondences supporting a vowel merger in the Ignaciano variety of Mojeño. Finally, I consider some implications for internal classification, advancing the hypothesis that Terena and Mojeño form a separate branch of the Arawakan family, the Achane branch, one that does not include Baure.