I have had a bit of luck with fastum regium. The Oxford Classical Text of Lindsay has fastum regium but his apparatus is sparse. He cites only one manuscript, but it is a pretty good one and is also very early (eight century, possibly). He does not offer any other readings. Lindsay was a very distinguished scholar of the history of Latin and his acceptance of the reading suggests it is to be taken seriously. The edition of the Etymologies in Brepols' classical texts series (which I mentioned in yesterday's post) is based on Lindsay's text. Reydellet's edition (Les Belles Lettres) gives a more detailed apparatus which cites manuscripts in three different major families. He also cites the fourth/fifth century grammarian Lactantius Placidus as saying Fastus superbia et est quarta declinatio. This suggests that there was perhaps some doubt about its declension. Perhaps the noun fastidium had some influence. On the other hand, no dictionary of later Latin or medieval Latin I could consult contained fastum -i in the sense of fastus here.
So, to wrap it up. If fastum regium is correct, then we must suppose that it is a very rare alternative form of fastus -us, here neuter and nominative and in apposition to benevolentia consulentis. It would then be translated as if it were fastus regius.
Or, as I said in yesterday's post, we can follow the reading in the Patrologia Latina edition, which has fastus regius and which, to me, has the strong support of the virtually identical phrase in Augustine. But this requires that corruption into fastum occurred very early in the manuscript tradition.
To minimize confusion, I would like you to change in your text the reading to fastus regius. That will be the reading I will use should the passage appear on the exam. You would then have to translate it as I did in the last paragraph of yesterday's post: "For regal pride was not the benevolence of someone consulting, but was the arrogance of a master."
