Idioms

let sleeping dogs lie
avoid mentioning unhappy past 
sick as a dog 
feeling very ill, vomiting a lot
go to the dogs , 
In the absolute pits
   a bad state
dog's life
  unhappy life
a dog's breakfast / dinner
  a badly done deed
(a case of) dog eat dog
  a situation in business or politics where there is a lot of competition and people are willing to harm each other for success
dog and pony show
  an event for publicity alone
shaggy dog story
  long joke with disappointing end
in dog house
    be in trouble
at a snail's pace
    move very slowly
put all your eggs in one basket
    stick on a single course of action for success
have egg on your face
    look very stupid
a fat cat
    an important person, often a rich person
a needle in a haystack
    a thing that is almost impossible to find
beat round the bush
    not describing accurately or exactly
hit the nail on the head
    describe or say something exactly or accurately
straddling the fence
    struggling to take decision
the grass is (always) greener on the other side (of the fence)
    about people who are not satisfied with what they have and always think that others have a better situation.
monkey around
    waste time by doing something unproductive
run against a clock
    a tight deadline (time is very short to complete a task or assignment)
(around / round) the clock
    all day and night without stop
as sharp as a tack
    Extremely smart
a babe in the woods
    a naive, innocent and  inexperienced person
as quiet as a mouse
    be very calm and quiet
pile on the (agony/gloom)
    make an unpleasant situation worse
make (a/your) pile
    make lot of money
don't count your chicken(before they are hatched)
    to be overconfident
count your blessings
    to be grateful for the things in your life
hot under the collar 
    angry or embarrassed
     Eg: He got very hot under the collar when I asked him where he had been all day.
have ants in your pants 
    excited or impatient
use a sledgehammer to crack a nut 
     use more force than what is necessary
a hard nut 
     a person difficult to deal with or to influence
a hard nut to crack
     a difficult problem or situation to deal with
chew the cud
     think slowly and carefully about something. Thinking and contemplating
         Eg: Stop chewing the cud and make a decision
go cap in hand ( to somebody)
     ask somebody for something, especially money, in a very polite way that makes you seem 
       less important        Eg: There is no other way. He will go cap in hand to his brother
a cat and dog life
     a life of disputes
bury (or hide) your head in the sand
     refuse to admit that a problem exists or refuse to deal with it
bury the hatchet
bury your differences
     stop being unfriendly and become friends again
apple of discord
bone of contention
     matter of dispute
blow your own trumpet
blow (or toot) your own horn
     praise your own abilities and achievements
bite your lip
     stop yourself from saying something or from showing an emotion
between the devil and the deep sea
       be in a dilemma
have several irons in the fire
      involve in many activities at the same time
set (put) a cat among the pigeons
      say or do something likely to cause trouble
Like a cat that has got the cream
      very happy
Let the cat out of the bag
       reveal a secret by mistake
be the cat's whiskers
       be the best person, thing or idea
       Eg: He thinks he is the cat's whiskers
On pins and needles
       be anxious
Like a cat on a hot tin roof
       be nervous
Pull your socks up
       improve performance
Pull a rabbit out of the hat
       bring a sudden solution .
Pull the wool over somebody's eyes
       Hide real intentions and to fool somebody .
Stick to your guns
       to refuse to change your mind about something even when other people are 
       trying to persuade you that you are wrong.
dyed in the wool
       Strong opinion or belief that never changes
make castles in the air
       wool gathering, to day dream
sail close to the wind
play with fire
       do something dangerous , rather illegal
trim one's sail
       to reduce your costs
fit as a fiddle
      in very good physical condition or health
a thorn in somebody's flesh
      a person or thing that repeatedly annoys somebody
be under fire  or come under fire
      to be criticized severely
         The minister for external affairs has come under fire from all sides.
like a cat on hot bricks
       very nervous
Cool as a cucumber
       very calm and composed especially in a difficult situation