Know Your Onions

A tempting but false theory credits this to C. T. Onions, a genuinely respected lexicographer who worked on the Oxford English Dictionary and later edited the Oxford Dictionary of English Etymology. The timing lines up just well enough to make the story attractive, but it's pure coincidence: there's no actual link between the real Mr Onions and the phrase. "Know your onions" is American in origin, first appearing in the early 20th century as just one of a whole grocery aisle's worth of similar expressions from the same era, including "know your oats," "know your apples," and "know your beans." Nearly all of them have vanished from use; "know your onions" is simply the one that happened to stick.