English Idioms and Idiomatic Expressions 

Idioms: Secrets and Discretion-1
from: 'behind closed doors'   to:  'let the cat out of the bag'

• 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."
    
    
• come to light
  • If details or facts come to light, they are revealed, exposed or become known.
    "The police say that no new evidence has come to light to help with the enquiry."
    
    
• 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."
    
    
• fishing expedition
  • If someone is on a fishing expedition, they are trying to obtain information or discover facts in any way possible, often using secretive or improper methods.
    "The lunch invitation was clearly a fishing expedition to obtain information about his private life."
    
    
• (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, a plan, or someone's identity, often unintentionally.
    "The actor hoped nobody in the hotel would recognize him, but the amazement on face of the receptionist 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!"
    
    
• keep the lid on
  • If you keep the lid on something, you hide it or control it to prevent people from finding out about it.
    "The company tried to keep a lid on the negotiations but word got out to the press."
    
    
• 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."