Poet Juvenal in his satires.
Contextually it referred to the notion that male guards should not guard women against sexual assault as they themselves are capable of committing such crimes, the only solution would be to have the women guarded by eunuchs.
Generally though it is considered the embodiment of the philosophical question as to where ultimate power should reside. It can be attributed as the basis, or at least the summarization, of the United States’ concept of division of political power. At any given moment a watchman is watched by two watchmen thus eliminating the possibility of corruption.
The phrase is also well known as the basis for the graphic novel, and film, Watchmen.
Synonymous phrases include:
Who will guard the guards?
Who will protect the protectors?
Who will police the police?
“But who will watch the watchmen? Themselves?”