set a thief to catch a thief

set a thief to catch a thief (third-person singular simple present sets a thief to catch a thief, present participle setting a thief to catch a thief, simple past and past participle set a thief to catch a thief)

1. To employ an unscrupulous person in order to catch another unscrupulous person.
   • 1866, Norman Macleod, editor, Good Words, page 415:

     The morality of setting a thief to catch a thief passes muster in the English force; but they do these things better in America. Pinkerton's system for the detective police in America is vastly superior to the British system.