Definition & Meaning of "[know] {one's} onions"

What is the origin of the idiom "know one's onions" and when to use it?

The origin of the idiom "know one's onions" can be traced back to the early 20th century, particularly in the United States. The word "onions" in this idiom is a metaphor for expertise or knowledge in a specific area. It likely originated from the idea that if someone knows how to select, grow, and use onions effectively in cooking, they possess a certain level of culinary expertise. This expression is used to praise or acknowledge someone's deep knowledge, skill, or expertise in a particular field or subject matter.