Curse not the king, no not even in your thought; and curse not the rich in your bedchamber: for a bird of the air shall carry the voice, and that which has wings shall tell the matter.
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Athanasius the Apostolic
If then those things, which are spoken in secret against you that are kings, are not hidden, is it not incredible that I should have spoken against you in the presence of a king and of so many bystanders? Defense Before Constantius
Generally one is advised not to say bad things about others. A curse is nothing else but a wish for something bad.…The word cautions against slander. Do not even have the intention, he says, of slandering someone—let alone actually slandering someone. Look at how great an evil this is: “Do not love speaking ill [of anyone], lest you be cut off.” Thus, he says the following: Not even in your thoughts, not even in your consciousness, should you “curse the king.” But if one is not supposed to curse anyone, the king should be cursed even less. ...
Said. Pigeons are taught to carry letters in the east, and Solomon alludes to this custom, or he makes use of this hyperbole to shew, that kings will discover the most secret inclinations by means of spies. We must not speak ill even of those who are worthy of blame, ver. 16. (Calmet)