je ne sais quoi 

French expression literally meaning "I know not what"; je I , ne...pas  not , sais know , quoi what . In Oxford English Dictonary since 1656. It is a way to express a quality that is hard to describe; a certain something.

Example: "dem bones is really stylish, he has a certain je ne sais quoi."

This is an entry in the mini series about French idioms in the English language.