A: Can you help me find someone that can fix my computer?

B: Sure. I recommend Sam from the finance department. He knows his onions about computers and everything.

Note: This is an American slang term that means “experienced in or knowledgeable about a subject.” Other 1920s variants of the idiom are “know one’s oil/oats/apples.” The only one that is still in common use is “know one’s onions.” Why onions? Well, explanations that relate the phrase to knowledgeable vegetable gardeners, or even to C. T. Onions, an editor of the Oxford English Dictionary from 1895, are just trying too hard. The 1920s America was a breeding ground for wacky phrases. Similarly, people also say “know one’s ropes,” a term from sailing which means “being informed about the details of a situation or task.”