Constantine Copronymus

The Cambridge Medieval History, Vol. IV: The Eastern Roman Empire (717-1453) (Cambridge: At the University Press, 1923), p. 11 (from Chapter I, by Charles Diehl):

Constantine V (740-775) has been fiercely attacked by the iconodule party. They surnamed him "the Stable-boy" (καβάλλινος) and "Copronymus" (named from dung), on account of an unlucky accident which, they said, had occurred at his christening.

On the nature of the "unlucky accident," see Theophanes, Chronicle, annus mundi 6211 = 718/719 A.D. (tr. Harry Turtledove):

In this year a still more impious son was born to the impious Emperor Leo: Constantine, the forerunner of the Antichrist. On December 25 Leo's wife Maria was crowned in the triklinos of the Augusteion. She went to the great church alone—without her husband—and without ceremony. She prayed before going in to the altar, then went into the great baptistery. Her husband and a few of her intimates preceded her in. While Germanos the chief prelate was baptizing Leo's successor (in both his evil and his rule) Constantine, the boy, because he was so young, gave a terrible, foul-smelling harbinger: he defecated in the holy font, as say those who were accurate eyewitnesses.