The English language often incorporates foreign words and phrases to create a more precise definition or to express a thought difficult to express in English alone. One example of this is the French expression je ne sais quoi, which literally translates into "I not know what." This expression attempts to define an indefinable quality which sets something apart from others. A fine painting such as Da Vinci's Mona Lisa, may have a certain je ne sais quoi that renders it more appealing than other paintings placed beside it.
A beautiful actress or model may also be said to possess a je ne sais quoi that makes her more captivating on film or generates more sexual appeal in a print ad. This star quality is not something that can be measured or defined objectively, but most people would agree there is indeed an X factor or intangible charisma at work. A renowned actor may not even be conventionally handsome, but still possess some unknown quality that elevates him to celebrity status.
Sometimes an intangible or undefinable quality of a fine wine or gourmet food can only be described as je ne sais quoi. Certain literary works or classical music pieces may also possess a certain artistic appeal which cannot be captured effectively in conversation. This is when the expression becomes especially appropriate, since the quality or aspect of a superior artistic work can defy description.
The use of je ne sais quoi can extend into other areas as well. A potential romantic partner may possess an intangible quality that makes him or her especially attractive or alluring. A charismatic instructor may also have a certain appeal that keeps his audience entranced as he delivers his lecture. Even a favorite pet may be said to have a je ne sais quoi that makes it stand out from the rest of the pack.
While there may be perfectly serviceable English expressions to describe an intangible quality, many people prefer to incorporate the French je ne sais quoi into conversation because it also has an intangible sense of elegance and panache. Casual use of familiar foreign expressions often indicates cultural refinement and a more extensive vocabulary. Sometimes a foreign expression captures or defines a concept more precisely or elegantly than its utilitarian English counterpart.