Q: I know the onion has many layers, but how did it get into the phrase “know your onions”?
A: The expression “know one’s onions,” meaning to be very knowledgeable or experienced about something, showed up in American English in the early 20th century.
The earliest example in the Oxford English Dictionary describes a horse with lots of experience at pulling a letter carrier’s mail wagon, as the one in this photograph from the Smithsonian National Postal Museum:
The OED citation, which we’ve expanded, is from a poem in the February 1908 issue of The Postal Record, a monthly journal of the National Association of Letter Carriers. In the poem, O. S. White, a letter carrier in Wilkes-Barre, PA, describes his workday. After a bit of grumbling about the demands of the job, he gets to his horse Billy:
But, never mind; Billy knows his onions,
He is not troubled with corns or bunions.
He travels along at a good, fair gait;
Unless the roads are bad, he is never late.
The dictionary’s first human example is from “The University Tongue,” a short story by Altha Leah Bass in the March 1922 issue of Harper’s Magazine.
When Ruth, a first-year college student, returns home for the holiday season, her mother asks if she has a good English instructor. Ruth replies, “Mr. Roberts knows his onions, all right.” Later, Ruth’s father says that parents, as well as students, can “learn their onions.”
The OED, in an entry for “know,” describes “know one’s onions” as a humorous colloquial play on an older use of the verb in various expressions meaning “to have learnt everything necessary about” a subject or “to be well informed” about it.
The dictionary’s citations for the older usage date back to the 1500s, but the early ones are relatively obscure. Here are a few clearer ones that we’ve found: “he knows his flock” (1621), “he knows his catechism” (1723), “he knows his business” (1744), “she knows her letters” (1799), and “they know their trade” (1800).
As for “know one’s onions,” the OED says it’s one of an assortment of offbeat expressions “used in same sense, but with substitution of a comically inappropriate noun, esp. the name of a vegetable or other foodstuff.” It adds that among such comic variations, the earliest and most common is “know one’s onions.”
Later versions of the usage cited by the dictionary are “knows his oil” (1924), “knows his cucumbers” (1929), “knew my okra” (1976), and “knows his carrots”—as in “It’s where every DJ who knows his carrots goes to be seen for the summer holidays” (Muzik Magazine, July 1995).
Note: Some language junkies have suggested that the usage may have been inspired by the name of the English lexicographer Charles Talbut Onions, better known as C. T. Onions. But that seems unlikely. Onions was a relatively obscure editor at the Oxford English Dictionary when the phrase first appeared across the Atlantic.