The French exclamation C’est inouï ! means “It’s incredible!”.
The word inouï [inwi] means unprecedented, incredible, unheard-of, extraordinary, amazing. It is a combination of the negative prefix in- and ouï, which comes from ouïr (to hear, to listen), from the Old French oir (to hear, listen), from Latin audiō (I hear, listen, pay attention), from Proto-Indo-European *h₂ew-is-d-, a compound of *h₂ewis (clearly, manifestly) and *dʰh₁-ye/o- (to render) [source].
A friend told me last week that the TGV (le train à grande vitesse), France’s high-speed rail service, is being rebranded the inOui. In fact, inOui is the new name, introduced in 2017, for certain premium services on the TGV. All premium services will be known as inOUi by 2020. The name Ouigo was introduced for discount TGV services in 2013 [source].
The name inOui has been mocked and criticised by many.
Ouigo works in English as well (We go), but I’m sure English speakers will be joking about inOui, if they aren’t already.
The trick is that ouïr is otherwise archaic/obsolete, inviting mockery even more readily than otherwise. This sets it apart from English unheard of or German unerhört “scandalous”.
Ouigo has to be a cross-linguistic pun.
I do know that as far back as the 90s ‘go’ could be used in slangy French. I had French friends who would say “On y go.” instead of “On y va.” for “Let’s go.” That probably helps more people get the pun in Ouigo.