E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
meant, originally, a fillet wound round the head. The diadem of Bacchus was a broad band, which might be unfolded so as to make a veil. Hieronymus, king of Syracuse (B.C. 216215), wore a diadem. Constantine the Great (306337) was the first of the Roman emperors who wore a diadem. After his time it was set with rows of pearls and precious stones. (Greek, dia-deo, to bind entirely.)