Meaning of the word "know your onions" in English

What does "know your onions" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

know your onions

US /noʊ jʊər ˈʌnjənz/
UK /nəʊ jɔːr ˈʌnjənz/
"know your onions" picture

Idiom

to be very knowledgeable about a particular subject or field

Example:
You can trust her advice; she really knows her onions when it comes to finance.
The mechanic quickly identified the problem, proving he really knew his onions.