This article contextualizes an old debate about the extent of knowledge of Latin by John Lydus, a state official and an erudite from sixth-century Constantinople, within a broader issue of the role of Latin in early Byzantium. It is argued here that Lydus’ startling etymological explanations had no relation to his level of knowledge of Latin, but reflected the declining official use of Latin in Byzantium by resurrecting the theory about Latin as a dialect of Greek.