unsayable

listen to the pronunciation of unsayable
Английский Язык - Английский Язык
Not capable of being said

The unsaid drives us to “speak;” but, in some ways, there are limitations on what we can say, because some part always remains unsaid — we must always attempt to say the unsayable.

Not allowed or not fit to be said

He was sacked, rather, for, saying the unsayable: for telling the truth.

{s} impossible to say; difficult to say (mainly because considered controversial or offensive)