The phrase "hitherto unthinkable" is correct and usable in written English.
It can be used to describe something that was previously unimaginable or impossible until now.
Example: "The advancements in technology have led to solutions that were hitherto unthinkable just a decade ago."
Alternatives: "previously inconceivable" or "once unimaginable."
Sometimes a startling, hitherto unthinkable move is needed.
With five rounds remaining they are closing on a hitherto unthinkable premiership.
And it has been at odds with Germany over the hitherto unthinkable: the need to write off some of the billions that Greece still owes.
Even the Sultan of Sokoto, the most powerful and respected Muslim religious figure in Nigeria, was pelted with satchels of water in the street — a hitherto unthinkable act of public humiliation.
They want to reduce the US government to the scale it was in the late 19th century, even if it involves the hitherto unthinkable idea that the US government may default on its debts.
This is not what Mrs Merkel means when she talks, with increasing fervour, of the need for "more Europe".Under pressure, Mrs Merkel may be ready to consider hitherto unthinkable steps, including transfers to the neediest countries and European-level taxes.
However, as well as maintaining pressure on traditional allies – an Israeli national security team arrived in Washington for talks with American officials yesterday – the situation may call for more radical partnerships that have hitherto been unthinkable, he says.
Within two or three sessions, my left arm, and hand, were beginning to do things that hitherto had been unthinkable.
Time for all of North Korea's neighbours to start thinking of how they might together deal with some of the unthinkables they have hitherto tried hard to ignore (see article).South Korea has responded vigorously to the results of an international investigation that proved North Korea sank the Cheonan, killing 46 sailors.
Rosenbaum's compositions have an absurdist dimension to them, as if Arnold Schoenberg had pooled resources with his former student John Cage: He loved hitherto-unthought-of, thereafter-unthinkable combinations of instruments.
AMID the vote-counting on May 1st 1997, Sir Malcolm Rifkind's election agent told him that Juniper Green, a hitherto rock-solid Conservative area of the Edinburgh Pentlands seat he had held for 23 years, appeared to have done the unthinkable.
