(Latin). The slave of slaves, the drudge of a servant. The style adopted by the Roman pontiffs ever since the time of Gregory the Great is Servus Servorum Die.
“Alexander episcopus, servus servorum Dei, Karissino filio Willielmo salutem.” —Rymer: Foedera, i. p. 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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