And variants (right arm, everything I have; far side; etc., etc.).
Oliver Wendell Holmes, Jr.? I have my doubts. I wasn't able to turn up so much as a promising initial clue via a Google Books search stripped down to the following: give simplicity "side of" complexity. Indeed, it raised suspicions the moment I first encountered it, as the sort of thing one might not even have said before 1935. After all, wouldn't a construction with something like naiveté (as in the fabled "second naiveté") have come first? (I could be quite wrong, of course.)