The original context is that a husband might lock his wife in the house to prevent her adulteries, but she is cunning and will start with the guards; hence, who guards the guards? The phrase has come to be applied broadly to people or organisations acting against dishonesty or corruption, esp. in public life. See Quis custodiet ipsos custodes? at Wikipedia.
Juvenal
Sed quis custodiet ipsos custodes?
Juvenal
There I also met the Spanish priest Antonio Llidó. He was accused of having hidden and protected people of the MIR who were persecuted. Antonio Llidó never denied this, saying that he could not lie to them. The guards would laugh at him, and commented that when Antonio Llidó was being tortured he was asked to name people, and he would say that he could not give them the names. 'And why not?' the guards asked. 'Because of my principles,' Antonio replied in his Spanish accent, which the guards mockingly imitated.
Antonio Llido
Kings and tyrants have armed guards wherewith to chastise certain persons, though they be themselves evil. But to the Cynic conscience gives this power—not arms and guards. (119)
Epictetus
It has been said a thousand times and in a thousand books that ancestor-worship is for the most part the source of primitive religions, and it may be strictly said that what most distinguishes man from the other animals is that, in one form or another, he guards his dead and does not give them over to the neglect of teeming mother earth; he is an animal that guards its dead.
Miguel de Unamuno
Up Guards and at them again.
1st Duke of Wellington
Juvenal
Juyi, Bai
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