quid pro quo

In the early 16th century, a quid pro quo was something obtained from an apothecary. That's because when quid pro quo (New Latin for "something for something") was first used in English, it referred to the process of substituting one medicine for another—whether intentionally (and sometimes fraudulently) or accidentally. The meaning of the phrase was quickly extended, however, and within several decades it was being used for more general equivalent exchanges. These days, it often occurs in legal contexts.