Plutarch De curiositate 16

[16] And indeed princes themselves—who are concerned to have as particular knowledge of all things as they can, and to whom it is in some sort necessary for the ends of government to maintain spies and intelligencers about them—are yet usually hated for nothing more than their retaining this lewd sort of people in quality of eavesdroppers of state and public informers. The first that employed this kind of officers was Darius in his younger years, when he could not confide in himself nor durst trust any one else. The Sicilian tyrants afterwards planted them in Syracuse; but upon a revolution that happened there, the people first fell upon these informers, and destroyed them without mercy. Of near affinity with these are common accusers, which, from a particular occasion imported in the word, were called sycophants, fig-blabbers; because, upon the prohibited exportation of that fruit, they became informers against those that broke this order. Much the like sort of people were those at Athens, where a dearth of grain happened and the corn-sellers were commanded to bring out their stores for public sale; and those that went about listening at the mills and prying into granaries, that they might find matter of information against offenders, were thence called aliterians or (if you please) mill-clackers. Which consideration, superadded to the rest that has been said, is enough to render this sort of malignant curiosity extremely execrable, and to be highly abhorred and most carefully avoided by every man who would desire, for mere reputation’s sake, not to be ranked among that profligate crew of villains which are looked upon as the most detestable of all mankind.