Je ne sais quoi is French for “I know not what.” In English, we use it to refer to an intangible, distinctive quality, especially of a person, that can’t be put in words—e.g., “I don’t know what makes him so charming. He just has a certain je ne sais quoi.” The phrase is often italicized and occasionally put in quotation marks.
The phrase has been in use in English for at least three centuries. In some old books, it is treated as a broader term representing various philosophical and aesthetic concepts not easily put in words, but by the 19th century je ne sais quoi was widely used the way it is today. A few historical examples are included below.
In French, the phrase is pronounced zhuhn-say-kwa, where zh represents the voiced j sound similar to the s in vision and the g in beige.
You will not however suppose that I am going to give a definition of this osmething, or nothing, this Je ne sais quoi, impressed upon the whole man by temper, habit, situation, and circumstance. [An Essay on Preaching, George Pike (1830)]
He was not a handsome man, but there was a je ne sais quoi in his manners and appearance that attracted my attention, flattered my imagination, and imperceptibly gained upon my heart. [The memoirs of the celebrated and beautiful Mrs. Ann Carson (1838)]
I was just going to tread on her foot, but a certain Je ne sais quoi, whether fear, shame, or love, held me back. [2001 translation of An Autobiography, Salomon Maimon (1888)]
[W]hile the real philosophers recognise and affirm the existence of a je ne sais quoi lying beyond natural phenomena, without being able to determine the nature of this je ne sais quoi, the artist, as if he were endowed with a separate sense, sets himself peacefully to work to realise, under this visible world, the second and wholly interior world. [The Influence of Baudelaire in France and England, G. Turquet-Milnes (1913)]
On the whole, the frontier just couldn’t afford ladies: only women, and women, at that, who had surrendered every last scrap of je ne sais quoi. [Texas Monthly (1977)]
As psycho killers go, Andrew Cunanan had a certain je ne sais quoi. [Salon (1999)]
Try as they might, the folks at Old Spice just couldn’t seem to recreate the viral je ne sais quoi of their best known ad campaign. [Gawker (2012)]