A student in my online course on the Markowitz Efficient Frontier model asked whether one could compute the correlation coefficient of returns on a bond and a stock, and another student replied that that would be comparing apples to oranges.

I wrote that apples and oranges are easily compared, and gave them this link to look at the chemistry / spectrometry of apples and oranges; in fact, they're quite similar. Thus, trying to compare the comparison of two very different items to the comparison of apples to oranges is like comparing . . . well, you get the idea.

All of this lead me to formulate this question: What's the origin of this idiom? I have my suspicions - having done no research whatsoever - and am interested in those of others.

If you know, tell us. If you research it first, please keep your research to yourself, or, at most, post a link without simply spilling the beans.

Which leads us to the question of the origin of another idiom . . . .