an open secret
something that a lot of people know, but do not talk about because it is supposed to be a secret

[TahlilGaran] Dictionary of Contemporary English ▲

secret known about by only a few people, who have agreed not to tell anyone else:
a secret meeting place
The details of the proposal must remain secret.
confidential used about information, especially in business or government, that is secret and not intended to be shown or told to other people:
a highly confidential report
Employees’ personal details are treated as strictly confidential.
classified used about information that the government has ordered to be kept secret from most people:
He was accused of passing on classified information to the Russians in the 1950s.
sensitive used about information that is kept secret because there would be problems if the wrong people knew it:
A teenager managed to hack into sensitive US Air Force files.
covert [only before noun] used about things that are done secretly, especially by a government or official organization:
a CIA covert operation
undercover [usually before noun] used about things that are done secretly by the police in order to catch criminals or find out information:
Detectives arrested the suspect after a five-day undercover operation.
underground an underground organization or newspaper is one that operates or is produced secretly and opposes the government:
Her father was a member of the underground resistance movement in France during World War II.
clandestine /klænˈdestən, klænˈdestɪn/ secret and often illegal or immoral:
clandestine meetings
his involvement in a clandestine operation to sell arms to Iran
a clandestine love affair
hush-hush informal used about information or activities that are kept officially secret:
He was put in charge of some hush-hush military project.
I’ve no idea what he does – it’s all very hush-hush.
have a secret
We have no secrets from each other.
know a secret (=about someone else)
You can tell Tom that I know his secret.
keep a secret (=not tell it to anyone)
Can you keep a secret?
tell somebody a secret
Shall I tell you a secret?
let somebody in on a secret (=tell them a secret)
Frank let me in on the secret.
reveal/divulge a secret formal (=tell it to someone)
He was accused of revealing state secrets.
give away a secret (=tell it to someone carelessly or by mistake)
I had to be careful not to give away any secrets.
share a secret (=tell it to someone because you trust them)
I trusted Alexander, so I decided to share my secret with him.
discover/find out a secret
He was afraid that someone would discover his secret.
a big secret (=an important secret or one that very few people know)
The event was supposed to be a big secret, but everyone knew about it.
a little secret (=a personal secret that very few people know)
You must promise me that this will be our little secret.
a closely guarded/well-kept secret (=a secret that few people are allowed to know)
The recipe is a closely guarded secret.
an open secret (=something that a lot of people know, but do not talk about because it is supposed to be a secret)
It was an open secret that he was having an affair.
a dark/terrible secret (=a secret about something bad)
I’m sure every family has a few dark secrets.
a dirty secret (=a shameful secret)
The exclusion of black people from the film industry is one of Hollywood’s dirty little secrets.
a guilty secret (=a secret that someone feels guilty about)
He had finally discovered Jo’s guilty secret.
sb’s innermost secrets (=very private or personal secrets)
She wasn’t confident enough to share her innermost secrets with him.
a family secret
Their normally strong relationship is threatened when he has to reveal a family secret.
a state/official secret (=a government secret)
He was accused of passing on state secrets to a foreign power.
a trade secret (=a company or business secret)
They must not betray their employer 's trust, for instance by giving away trade secrets.
military secrets
He was sent to prison for five years in 1933 for selling military secrets to Germany.

[TahlilGaran] English Thesaurus ▲

open secret
n. Something that is supposed to be a secret but that everyone knows.
It is an open secret that Mary and John are engaged.
Who will be appointed as the next president of the college is an open secret.

[TahlilGaran] English Idioms Dictionary ▲