Know one's onions

    懂行     

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

    B: Sure. I recommend Michael from the financial 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 your oil/oats/apples." The only one that is still in common use is "know your onions." So, 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, like today's China.