A colleague writes to object strenuously that it would have been anachronistic to refer to the Lakhmids as malkia arbaiia “Kings of the Arabs” during the Sasanid era, as the Syriac term ܥܪ̈ܒܝܐ ˁarbāye is never applied to Arabs, the terms ܛܝ̈ܝܐ ṭayyāye and ܣܪ̈ܩܝܐ sarqāye being employed in its place. Therefore, the text can only date from a much later period, in which it became common to refer to the Arabs as ˁarbāye in Syriac.
Leaving aside the obvious objection that Mandaic is not Syriac, if we really want to know how the Lakhmids styled themselves, we need look no further than the 4th century funerary inscription of the Lakhmid king Imru’ al-Qays (r. 295-328) from al-Namāra:
Our inscription famously begins تي (هذه) نَفسُ (شاهدة قبر) امرؤ القيس بن عَمرو مَلِكُ العرب, “this is the funerary inscription of Imru’ al-Qays, son of Amr, King of the Arabs.” It provides us with a direct witness to the fact that the Lakhmids styled themselves the “Kings of the Arabs,” of which the Mandaic phrase malkia arbaiia is an obvious calque.
Why were the Lakhmids ˁarbāye and not ṭayyāye? They formerly inhabited the region known as Arbāyistān/Bet ˁArbāye in northern Mesopotamia (stretching from Nusaybin to the Tigris, from Cizre in the north to Jabal Shinjar in the south), until they were dislodged by the Sasanids and migrated south to the area in which they encountered the Mandaeans. It was there they founded al-Hira, and from there they ruled until they were briefly expelled by the Mazdakite al-Harith al-Kindi, only to return and reign for an additional 71 years—exactly according to the chronicle in the Great Treasure.