Idioms not from America

Is it okay to use an idiom not from America in my essay? I’m thinking probably not because the admissions people may not know what it means. I did not even know what it means until I looked it up, but it is from England, so it’s not like it’s Japanese or something.

It would help to know what it is. Plenty of educated people are familiar with idioms from another country… and admissions people are all well educated.