The phrase to know one’s onions means to be very knowledgeable about something.
It originated in American English in the late 19th century with onion in the singular. The earliest instance that I have found is from The Philadelphia Inquirer (Philadelphia, Pennsylvania) of Wednesday 15th June 1898:
It is laughable how some crews are trying to hide their true form. The knowing babies, after passing Turtle Rock, begin to row in wretched style, sufficient to make the angels weep. A weakness among the “want-to-bes” is to stop and take a rest above Turtle Rock and then come down to the dam at a terrible rate. You must “know your onion” to get your money on right.
The author of the following recipe, published in The Pittsburg [sic] Press (Pittsburgh, Pennsylvania) of Wednesday 31st December 1902, punned on the phrase:
In cooking onions it is well to first know your onion. Is it mild, strong, new or old? Whatever, whichever it is, it may be evolved to deliciousness by the cook in its preparation if wisely performed. A new summer onion or a very mild winter one is never so acceptably presented as when simply boiled in salted water until perfectly tender through and through, well drained, and dressed with butter, pepper and salt. Onions should be put on to cook in boiling water, and this water changed after fifteen minutes to fresh, boiling salted water.
There is also a pun in this paragraph from the Republican News Item (Laporte, Pennsylvania) of Thursday 25th June 1903:
The Experimental farmer comes down strong on the butter business, and when he talks about leeks, we have nothing more to say in advice, for it is plain to see that “he knows his onion.”
The origin of this phrase is unknown. One theory is that onions is short for onion rings, rhyming slang for things. But, apart from the fact that this does not account for the original instances with onion in the singular, no occurrence of to know one’s onion rings has ever been recorded.
The following from The New York Times (New York, N.Y.) of Sunday 7th June 1903 is interesting because it shows that very early the origin of the phrase seems to have been lost and that there was an additional meaning, to mind one’s own business:
“He Knows His Onion.”
To the Editor of The New York Times:
Apropos of the discussion of the meaning and origin of “makes no bones,” will some slang etymologist let us know the origin and connection of “he knows his onion,” meaning he minds his own affairs, or is posted about certain pertinent situations
New York, June 5, 1903.
Curiously, since the late 19th century as well, there has been in French a colloquial use of the plural oignons (onions) corresponding to the sense to mind one’s own business of to know one’s onion: the phrase c’est mes, tes, etc., oignons or ce sont mes, tes, etc., oignons (literally it is my, your, etc., onions or these are my, your, etc., onions) means it is my, your, etc., own business; the negative c’est pas mes, tes, etc., oignons or ce ne sont pas mes, tes, etc., oignons means it is none of my, your, etc., business; and s’occuper, or se mêler, de ses oignons (literally to mind one’s own onions) means to mind one’s own business.
This figurative use is first recorded as c’est mes oignons in Déception (Disappointment), from Les Soliloques du Pauvre (The Poor Man’s Soliloquy – Paris, 1897), by the French poet Gabriel Randon de Saint-Amand (1867-1933), who, under the pen name of Jehan Rictus, wrote in the popular language of Paris:
Mais j’ t’aim’ comm’ ça…. c’est mes z’ognons
Et tout l’ reste il est d’ la gnognotte !
But I love you as you are… it’s my onions
And all the rest is small beer!
Like English to know one’s onions, this French use of oignons is of unknown origin.
One theory is that this sense of oignons originated in the obsolete phrase—itself of unknown origin—il y a de l’oignon (literally there is some onion), meaning there are obscure motives, suspicious events, which give an inkling that difficulties are afoot (cf. a snake in the grass): the sense hidden matter of oignon would have evolved into personal affair.
It is more likely, as Sylvie Claval and Claude Duneton (1935-2012) write in Le Bouquet des expressions imagées (Robert Laffont, 2016), that s’occuper de ses oignons originally meant s’occuper de ses pieds (to mind one’s own feet) because, during the second half of the 19th century, the forms ognons, ognes, oignes were used in the sense of feet.
The phrase s’occuper de ses pieds is in turn a euphemism for s’occuper de ses fesses, or de son cul (literally to mind one’s own buttocks, or one’s own arse), so that s’occuper de ses oignons may also allude to oignon (brûlé) (literally (burnt) onion) in the figurative sense of anus.