in law, q. d. "what for what," denotes the giving one thing of value for another; or the mutual consideration and performance of both parties to a contract.

Quid pro quo, or Quid pro quo, is also used in physic to express a mistake in the physician's bill, where quid is wrote for quo, i.e., one thing for another; or of the apothecary in reading quid for quo, and giving the patient the wrong medicine. Hence the term is in the general extended to all blunders or mistakes committed in medicine, either in the prescription, the preparation, or application of remedies.