The meaning of the idiom “it takes one to know one” is that the person who expressed criticism has similar faults to the person being criticized. The phrase is used as a retort to an insult, implying that the accuser is guilty of the same thing they are accusing of. The phrase can be used as an insult or playfully, depending on the context and the relationship between the speakers.
The origin of this expression is unclear, but some sources suggest that it dates back to the 15th or 16th centuries. Some think that the best or better foot refers to the right foot, because the left foot was considered unlucky. For example, Shakespeare wrote in King John: “Nay, but make haste; the better foot before.” A near equivalent is the proverbial it takes a thief to catch a thief, meaning “no one is better at finding a wrongdoer than another wrongdoer.“ The phrase was popularized by the science fiction writer Robert A. Heinlein in his novel The Moon Is a Harsh Mistress, and by the economist Milton Friedman in his book There’s No Such Thing as a Free Lunch.
