English Idioms and Idiomatic Expressions 

Idioms: Secrets and Discretion-2
from: 'like a thief in the night'   to:  'the truth will out'

• 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 the 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?"
    
    
• stick one's nose (in) where it's not wanted
  • If someone sticks (or pokes) their nose in where it’s not wanted, they show too much interest in something that is not their business, or become involved in an intrusive or nosey manner.
    "That man is really annoying! He’s always sticking his nose in where it’s not wanted !"
    "The manager has no right to go sticking his nose into my personal life!"
    
    
• 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."