behind closed doors

If something takes place behind closed doors, it is done privately or in secret, with no observers or intruders.
“The matter was discussed behind closed doors.

bite your tongue

If you bite your tongue, you try not to say what you really think or feel.
“It was difficult for me not to react; I had to bite my tongue.”

cover your tracks

If you cover your tracks, you conceal or destroy evidence of what you have been doing or where you have been.
“Charlie was sorry he hadn’t covered his tracks better when his wife discovered he had been unfaithful.”

(a) dark horse

If you refer to someone as a dark horse you mean that they are secretive or that little is known about them.
“I can’t say I know my neighbour. He’s a bit of a dark horse.”

in the dark

If someone is kept or left in the dark about something, they are not informed about it.
“The personnel was kept in the dark about the merger until the last minute.”

(as) dumb/close as an oyster

Someone who is as dumb or as close as an oyster will never reveal something told in confidence or betray a secret.
“Sophie will never repeat what you tell her. She’s as dumb as an oyster.”

(a) fly on the wall

A person who discretely watches a situation without being noticed is called a fly on the wall.
“I’d like to be a fly on the wall when the management discusses my project.”

give the game away

If you give the game away, you reveal a secret or a plan, often unintentionally.
“He hoped nobody in the hotel would recognize him, but an employee gave the game away.”

good walls make good neighbours

This expression means that respecting one another’s privacy helps create a good relationship between neighbours.
“We try not to disturb the people next door. Good walls make good neighbours!”

hold your tongue

If you hold your tongue, you stay silent and say nothing.
“Harry was of a different opinion but he decided to hold his tongue.”

keep a low profile

A person who keeps a low profile tries not to attract public attention.
“The inventor is a discreet man who keeps a low profile.”

keep under your hat

To keep something under one’s hat means to keep a secret.
“My boss has promised me a promotion, but it’s not official yet, so keep it under your hat.”

keep under wraps

If something is kept under wraps, it is held secret and not revealed to anyone.
“The plan was kept under wraps until the contract as officially signed.”

let cat out of the bag

If you let the cat out of the bag, you reveal a secret, often unintentionally.
“When the child told her grandmother about the plans for her birthday, she let the cat out of the bag.”

like a thief in the night

Someone who acts like a thief in the night does something secretly or in an unexpected manner.
“He left the company like a thief in the night, without telling his colleagues or saying goodbye.”

lips are sealed

If you say that your lips are sealed, you  promise not to reveal a secret.
“I promise I won’t tell anyone. My lips are sealed.”

mum’s the word

To say ‘mum’s the word’ means that the subject or plan is a secret and must not be revealed.
“We’re organizing a surprise birthday party, so mum’s the word – OK?”

off the record

If you say something off the record, you do not want anyone to repeat it publicly.
“His comment was made off the record, and shouldn’t have been published.”

on the QT

Something that is done on the QT (shortened form of ‘quiet’) is done quietly or discreetly.
“They got married on the QT last summer and told nobody.”

on the sly

If you do something on the sly, you do it secretly or furtively.
“He made such quick progress that the others suspected him of having private lessons on the sly.”

off the record

Information given off the record is not yet official or public.
“It’s not official yet but off the record John Brown is going to be appointed head of our department.”

(an) open secret

A fact that is supposed to be a secret but is actually widely known is called an open secret.
“It’s an open secret that Paul and Emily are living together.”

play your cards close to your chest

If your play your cards close to your chest, you keep your plans, intended actions or tactics secret from other people.
“He’s a very cautious businessman who plays his cards close to his chest.”

(as) quiet as a mouse

When someone is as quiet as a mouse, they make no noise at all.
“The burglar was as quiet as a mouse as he moved around the house.”

(a) skeleton in your closet

A skeleton in the closet is an embarrassing or shameful secret that is kept hidden because it would cause problems if it were revealed.
“The candidate seems perfect. Let’s hope he doesn’t have any skeletons in the closet.”

spill the beans

If you spill the beans, you reveal a secret or talk about something private.
“Come on! Spill the beans! What did he say?”

in strict confidence

If you say something in strict confidence, you tell it as a secret not to be revealed.
“Please don’t repeat this. I’m telling it to you in strict confidence.”

sweep under the rug

If you sweep something under the rug (or carpet), you try to hide it or keep it secret because it is embarrassing.
“They tried unsuccessfully to sweep the scandal under the rug.”

(a) trade secret

The term ‘a trade secret‘ refers to the secrecy of a company’s production methods but is often used teasingly.
“Can you give me the recipe for your lemon meringue pie?”  ” No way – that’s a trade secret!”

the truth will out

The expression ‘truth will out‘ means that despite efforts to conceal the facts, the truth cannot be hidden forever.
“I don’t know if the police gave the full details, but inevitably truth will out.”