What does 'Je ne sais quoi' mean in French?

Je ne sais quoi is a French expression which means, literally, 'I don't know what'. It's one of those useful little expressions you'll hear a lot - even in English - and it can be used in a few different ways.

There are a few different scenarios in which you can use the expression Je ne sais quoi.

1. As a phrasal noun referring to something you can’t quite express, often something a little special

Il a un certain je ne sais quoi. - 'He has a certain something about him.'

Il y a un je ne sais quoi dans ce plat. - 'There's a certain something in this dish, it's really tasty.'

2. ‘Or something/Whatever’

Ce film parle d'un magicien ou je ne sais quoi. - 'The movie’s about a wizard or something.'

3. In front of an adjective. When used in this way, we add de + an adjective after the phrase.

Il y a je ne sais quoi d'étrange dans cet endroit... - 'There’s something strange about this place…'

While je ne sais quoi is the only one of these expressions regularly used in English, when speaking French, you can also use the following expressions in much the same way, although they are not used as commonly as je ne sais quoi:

on ne sait quoi

Dieu sait quoi

Sometimes, particularly when used as a noun, je ne sais quoi is also spelt with hyphens, as in

Je-ne-sais-quoi

You can also add in 'pas' when using the phrase in a more literal sense, though this is considered more informal:

Je veux manger je ne sais pas quoi. - 'I want to eat something, I don’t know what.'

Usually, however, we use the ne litteraire with this phrase, meaning you drop the pas.

The first recorded usage of the phrase Je ne sais quoi we have in English can be found in a 1656 dictionary of ‘hard words’ and loanwords by Thomas Blount known as Glossographia, where it is spelt Je ne scay quoi, and defined as 'four French words, contracted as it were into one, and signifies I know not what.'

Written records of the phrase exist in French dating from the early seventeenth century.