Tunen is a Bantu (Niger-Congo) language spoken in Cameroon in the Centre and Littoral provinces, with Guthrie classification A44 (Maho 2003, 2009). The language is typologically unusual in displaying SOV base word order, i.e. a head-final verb phrase (O V), while elsewhere being head initial (prepositions, Dem-N order). Dugast (1971) and Mous (1997) note that cardinal numerals appear postnominally (N NUM), with the exception of the form -mɔt̀ ɛ́ ‘one’, which can also appear before the noun in the plural. In this article, I investigate this puzzling prenominal case of -mɔt̀ ɛ́ and argue based on new data that it is not synchronically a numeral and has in fact grammaticalised to function as an indefinite determiner marking epistemic specificity. I use controlled elicitation to show semantic tests to support this and present syntactic arguments that prenominal -mɔt̀ ɛ́ appears in the position of a determiner rather than a numeral. I then investigate the marker’s likely grammaticalisation over time by considering the “seemingly universal” grammaticalisation path of numeral ‘one’ to (specific) indefinite markers that has been proposed in the typological literature (Givón 1981: 35); Heine (1995, 1997). I test the predictions of such an account by means of a corpus study of 61 folk tales (contes) published in Dugast (1975), and conclude by a brief survey of related Cameroonian languages.