This article aims to make a contribution to the debate concerning the origins of the Moroccan Arabic genitive preposition d(yal) and its implications for the emergence of Moroccan Arabic recently reignited in Heath (2015). The latter sources the preposition to a combination of Late Latin allative dē and pronouns in the context of a language shift that took place in Roman cities in the Maghreb. This hypothesis is shown to be inconsistent with both the linguistic and historical evidence, which favour the alternative hypothesis that the preposition arose from merger between the Old Romance genitive preposition de/i and the Classical Arabic definite article ( ʔ ) al (Ouhalla 2009a&b). This development took place in the context of Andalusi Arabic, which emerged in Spain in the ninth and tenth centuries and subsequently spread to Morocco by migration. In addition to outlining further linguistic evidence for the hypothesis, the article highlights the role of diglossia in the emergence of Andalusi Arabic, where Classical Arabic was the High variety that accounted for much of the vocabulary and Iberian Old Romance as the Low variety that accounted for the syntax base.