je ne sais quoin.

1656 T. Blount Glossographia Je-ne-scay-quoi, four French words, contracted as it were into one, and signifies I know not what, we use to say they are troubled with the Je-ne-scay-quoy, that faign themselves sick out of niceness but know not where their own grief lies, or what ayls them.

1671 J. Aubrey Countrey Revell ii. iii Seemed to give a mournefull je n'scay quoy.

1696 T. D'Urfey Comical Hist. Don Quixote: 3rd Pt. iv. ii. 38 Some sweet alluring Jen Scay Quoy, Some pleasing pretty tickling Toy.

a1734 R. North Examen (1740) iii. viii. §14 592 Now this Word Post has a je ne sçai quoi Sound of a deep Design.

1745 Gentleman's Mag. June 324/2 So refined a Je-ne-scay-quoy was about 'em, For goddesses there was no reason to doubt 'em.

a1777 S. Foote Cozeners (1778) iii. ii. 72 There is, besides, an elegance, a je ne scai quoi, in your son's air.

1881 W. S. Gilbert Patience ii. 35 A..Je-ne-sais-quoi young man.

1894 E. Sullivan Woman 71 Her famous nepenthe was simply the irresistible fascination of her ‘Je ne sais quoi’.