hush-hush[adjective]Synonyms: secret, classified, confidential, restricted, top-secret, under wraps
English Thesaurus: secret, confidential, classified, sensitive, covert, ... [TahlilGaran] English Synonym Dictionary ▲
hush hush adj.,
informal Kept secret or hidden; kept from public knowledge; hushed up; concealed.
The company had a new automobile engine that it was developing, but kept it a hush-hush project until they knew it was successful. [TahlilGaran] English Idioms Dictionary ▲