It is widely believed that the Rbbl bn Hfʿm grave inscription found at Qaryat al-Fāw is the earliest example of Old Arabic. The ten-line inscription – written in the Sabaic script – attests the common Arabic definite article, ʾl, plus several other non-Sabaic linguistic features. I argue that the definite article is not a suitable diagnostic of genetic affiliation, and other features, such as mimation, the conjunction ʿdky, and more, should also be given consideration. Through a close linguistic examination based on the principle of shared morphological innovations, I demonstrate that none of the morphological innovations which characterize Arabic are attested in this inscription. As such, its language is probably not a descendant of proto-Arabic. Our results further suggest that the ʾl- article, which has previously been used as a marker of Arabic, was simply one of many definite article forms which spread to Arabic, and other Semitic languages of Arabia, through areal diffusion.