Meaning

To be experienced in or knowledgeable about a subject.

P1000013

Origin

The English grammarian and lexicographer C. T. (Charles Talbut) Onions was an editor of the Oxford English Dictionary from 1895 and continued to write reference works throughout a long and distinguished career. His last work was The Oxford Dictionary of English Etymology (1966), which was published a year after his death. If I knew as much etymology as he did I could certainly claim to 'know my onions', and it is tempting to assume that this is where the phrase originated.

While it is true that 'know your onions' was coined at a time when C. T. Onions had established a reputation (i.e. during the 1920s), the match between the phrase and his name is just a coincidence. Know your onions is in fact an American phrase. There are many references to it in print there from the 1920s onward, but none in the UK or elsewhere until the middle of the century. For example, this from Harper's Magazine, March 1922:

"Mr. Roberts knows his onions, all right."

Another example comes soon afterwards, in The Lima News, May 1923:

"The Columbus statement declares that the league is ready to give the Donahey boom every aid and comfort. Of course! Why not? Governor Donahey knows his onions..."