I was showing Sean an algebraic solution to a math question, and at the end of it, right next to the answer, I added QED. I don't know who put this in my head, but I had always thought QED was the acronym for Quite Easily Done. And I asked Sean if he knew what QED stood for.
His eyes lit up and he said, "Well, some people say it stands for Quite Enough Done, but actually, it is Latin or maybe Greek for quod erat demonstrandum, which means "that which was to be demonstrated."" My jaw almost dropped :)
Found this on wikipedia:
"The phrase quod erat demonstrandum is a translation into Latin from the Greek ὅπερ ἔδει δεῖξαι (hoper edei deixai; abbreviated as ΟΕΔ). Translating from the Latin into English yields, "that which was to have been demonstrated"; however, translating the Greek phrase ὅπερ ἔδει δεῖξαι produces a slightly different meaning. A better translation from the Greek would read, "precisely what was required to be proved." The phrase was used by many early Greek mathematicians, including Euclid and Archimedes. These mathematicians, in particular Euclid, are credited with founding axiomatic mathematics with its emphasis on establishing truths by logical deduction (rather than experimentation or assertion); their use of this phrase symbolizes this emphasis, as well as marking this important step in the development of mathematical philosophy."
And where did Sean get this ancient nugget of knowledge? No where else but from the wonderful Kjartan Poskitt's Essential Arithmetricks. Gotta love the Murderous Math books.