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Under-the-table payment
A secret, hidden action / transaction
Under a cloud
Under suspicion due to a serious mistake
Under construction
In the process of being built
Under fire
Being criticized or held responsible
Under new management
Controlled by a different person than before
Under canvas
Living in a tent
Under arms
Provided with weapons
Under duress
Under threat or force
Under age
Minor, not of legal age
Under investigation
Under interrogation due to charges
Red-letter day
An important day
Red tape
Excessive or unnecessary rules
Caught red-handed
To discover someone while doing something illegal
Red herring
Something that distracts from the real problem
Like a red rag to a bull
Certain to produce an angry reaction
Made me see red
To become very angry
Show the white feather
To show cowardice
White elephant
Something that costs a lot of money
White lie
A small lie told to avoid hurting someone
Black sheep
The outcast
Get out of bed on the wrong side
To be in a bad mood for the day
Hit the nail on the head
To be exactly right
See eye to eye
To come to the same opinion
Kill two birds with one stone
Achieving two aims at once
Put your foot in something
To embarrass someone unintentionally
Finger in too many pies
Being involved in too many things
Bone to pick with somebody
To be annoyed with someone and want to talk
Feel something in your bones
To believe something strongly without explanation
Put the cart before the horse
To do something in the wrong order
Get your teeth into something
To become fully involved
Keep an open mind
To wait for all facts before judgement
Take the law into your own hands
To punish instead of waiting for legal action
Turn the tables on somebody
To cause a complete reversal of circumstances
Pay somebody back (in his own coin)
To treat someone as they treated you
Put all your cards on the table
To not hide anything and be honest
In a person's good or bad books
If you are in a person’s good book they are pleased with you if you are in their bad book they are not pleased with you
Give somebody a wide berth
To avoid or stay away from someone
Get more than you bargained for
Receiving something unexpected, usually worse
Make short work of something
To finish something quickly and easily
Be out of step with somebody
To have different beliefs or opinions.