To “know your onions” means knowing a lot about a subject. It’s a phrase that isn’t so common anymore. It’s a child of 1920’s slang, a slang that dreamed up such gems as “the bee’s knees”.
This is perhaps one of the stranger idioms you will find. What do onions have to do with being smart, anyway?
It all starts with a man with the unfortunate last name of ‘Onion’. English language expert Charles Talbut Onions edited the Oxford English Dictionary from 1895 through the mid-20th century. C. T. Onions knew his stuff where the English language was concerned, which creates the possibility that has name alone was enough to get the phrase going.
But there was more than one Onions. Mr. S. G. Onions of the numismatic industry produced coins for English schools starting in 1843. These coins were not used as real currency, but instead as learning tools for students learning to count. They had inscriptions that explained how currency added up, similar to “60 cents make a dollar” and so forth.
However, the first print appearance of “know your onions” didn’t occur until the 1920’s – in the U.S., far from either Onions’ lineage. The fact that the phrase seemed to first pop up in America suggests that neither of the Onions had a hand in its evolution.
Similar phrases, like “know your apples,” were created in the 1920’s, but only onions stuck around.
The idiom also makes for a great song.