Scholars of Arabic dialects have long noted the occurrence of a morpheme in a widespread number of dialects, realized -ən or -an, frequently suffixed to morphologically indefinite nouns, especially when followed by an adjective. Separately, another morpheme, realized -un or -u, is attested with a slightly different distribution in the dialects of western Yemen. Traditionally, scholars have interpreted both morphemes as reflexes of an etymological case vowel + tanwīn, traditionally labeled “dialectal tanwīn.” In this paper, I offer a new reconstruction (...) of the origin and diachronic development of this morpheme. Throughout I integrate data and insights from comparative Semitics, as well as recently studied pre-Islamic epigraphic and textual materials, in order to break the familiar Classical Arabic / dialectal Arabic dichotomy and reframe the way in which historiography of features in the dialects is conducted. (shrink)