The word “onion” (cebolla) you may know. You may have eaten “onion rings” (de cebolla rebozadas) and onion soup.
Here the interest is in two idioms that use “onion”. Examine these:
- “Mr. Morales knows his onions”.
This means that he knows his subject well, that he is very knowledgeable in the subject. The origins of this idiom comes from the editor of the Oxford English Dictionary from 1895. His name was Mr. Charles Onions and he was considered to know his subject (the English language) very well. Hence the idiom.
Perhaps in Spanish, it is to “saber mucho sobre algo o ser experto en algo”.
- “In Brussels Mrs. May felt like “a lonely little petunia in an onion patch”.
This idiom means that she felt out of place or did not fit in with the European leaders. A similar expression, in Spanish, might be “estar como pez fuera del agua”, to be like a fish out of water. An “onion patch” would be the area in your vegetable garden where you grow your onions.
So, before your speak about something “know your onions”, in order to avoid feeling out of place, or feeling “like a petunia in an onion patch”.