In 2005, I asked Merriam-Webster the following question. Assistant Editor, Emily A. Vezina, replied as follows.
Q: Your dictionary defines “QED” as “which was to be demonstrated.” I don’t know what this means, so I found two examples. Unfortunately, neither helped; both seem to imply different meanings of the term. Can you clarify?
- “Ann Coulter is a vitriolic right-wing pundit. . . . Tom Frank is a vitriolic left-wing pundit. QED: Tom Frank must be like Ann Coulter.”
- “Resolution 1441, unanimously passed by the Security Council, ordered Saddam to make full accounting of his W.M.D. program and to cooperate with inspectors, and warned that there would be no more tolerance for concealment or obstruction. Kay’s finding of ‘dozens of W.M.D.-related program activities,’ concealed from U.N. inspectors, constitutes an irrefutable material breach of 1441—and open-and-shut vindication of the U.S. decision to disarm Saddam by force. QED.”
A: “QED” is an abbreviation of the Latin phrase that literally means “which was to be demonstrated” or “which was to be proved.” It is sometimes written at the end of an argument (or a mathematical proof) to say, “This is the end of my argument, and see, I proved what I set out to prove.”
In your first example, “QED” means “therefore I have proven that Tom Frank must be like Ann Coulter.”
In the second example, QED means, “There’s my argument. There’s my proof. I proved what I set out to prove.”
The abbreviation has no succinct exact translation in English, since “QED” often serves more of a function than having an actual translation, but it is used to indicate the end of an argument where the arguer believes he has proved his point.