Q From Gary Pentland, UK: A group of friends were talking about work and someone said ‘He knows his onions’, meaning that he knows all about a particular subject. One of the gang asked where it came from and I said I would find out, as I know this wonderful Web site that’ll let me know. Imagine my dismay when, checking it out this morning, I couldn’t find it. So, over to you. Where does know your onions come from?
A Sorry to have disappointed you. Though the site is getting big, it sometimes surprises people to learn that I’ve not yet written about every word and phrase in the language. Let me repair this particular omission without further delay.
As it happens, I am writing this reply having a few minutes ago heard a BBC radio programme, You and Yours on BBC Radio 4, give an answer to this question. Based on e-mails from a couple of listeners, the presenters said that it came from the name of the noted British lexicographer and grammarian C T Onions, who worked on the Oxford English Dictionary (he wrote the last entry in the first edition of that monumental work, a cross reference for zyxt, a Kentish dialect form of part of the verb to see). His best known work is the Oxford Dictionary of English Etymology, published in 1966, a year after his death. The presenters said Onions was so well regarded that he became the epitome of the well-regarded expert.
Don’t believe a word of it. However erudite a lexicographer C T Onions was, he certainly wasn’t the source of the saying. No matter that he was eminent in his own field, he never gained the kind of wide public recognition that would have caused such a phrase to gain and retain recognition (I find to my chagrin that this is often the case for writers on word histories).
The crucial fact is that the expression isn’t British but American, first recorded in the magazine Harper’s Bazaar in March 1922. It was one of a set of such phrases, all with the sense of knowing one’s stuff, or being highly knowledgeable in a particular field, that circulated in the 1920s. Others were to know one’s oats, to know one’s oil, to know one’s apples, to know one’s eggs, and even to know one’s sweet potatoes (which appeared in a cartoon by T A Dorgan in 1928). You may notice certain similarities between the substances mentioned, most being foods and most having names that start with a vowel.
They contain much of the verbal inventiveness and mildly juvenile wordplay that characterises another American linguistic fad of the flapper period, that of describing something excellent of its kind in terms of an area of an animal’s anatomy (elephant’s instep, gnat’s elbows and about a hundred others — see my piece on bee’s knees for more).
As with bee’s knees, one of these multifarious forms eventually triumphed and became a catchphrase that has survived to the present day. Sadly it has provided an opportunity for a couple of people to provide what seems like an erudite explanation but which is really no more than a popular etymology.