1) Water Under The Bridge-something that has happened and cannot be changed, hence, not important-
I should have negotiated a higher salary when I was offered the job, but that's water under the bridge now.
2) Skeleton In The Closet-a hidden, shocking or shameful secret -
Even the most innocent looking man has skeletons in his closet.
3) Icing On The Cake-something that makes a good situation even better-
George was happy to have his first book published. All those congratulatory messages and fan mail that came in were the icing on the cake.
4) Have A Soft Spot For Someone-have special feelings for someone-
She had a soft spot for her youngest brother and was always fighting for him, even if it was his fault.
5) Head Over Heels-fall deeply and completely in love, especially suddenly-
Tom and Mary are head over heels in love with each other and are going to get married next month.
6) Flash In The Pan-something that is promising in the beginning but fizzles out later-
His initial good performances were a mere flash in the pan. Nowadays, he delivers only mediocre work.
7) Down To The Wire-a situation where the outcome is not decided until the very end-
With both teams being equally matched, the match was very exiting and went down to the wire.
8) Break The Ice-to initiate social interchanges and conversation, to get something started-
I tried to break the ice by talking to the people next to me about the weather.
9) Born With A Silver Spoon In Mouth-to be born to parents who are rich and have a good social rank, someone who is born into privilege and wealth-
He does not need this job as much as I do, he is born with a silver spoon in his mouth.
10) Bite The Bullet-to endure a painful or otherwise unpleasant situation that is seen as unavoidable-
I hate going to the dentist, but I'll just have to bite the bullet.
11) Let The Cat Out Of The Bag-to allow a secret to be known, usually without intending to-
Joey was trying to keep the party a secret, but Rachel went and let the cat out of the bag.
12) The Final Nail In The Coffin-an event which causes the failure of something that had already started to fail-
This latest evidence could be the final nail in the coffin for Jackson's case.
13) An Eye For An Eye-retributive or vengeful justice, retaliation, punishment in which an offender suffers what the victim has suffered-
Some guy took his wife, so he took the wives of others, it was an eye for an eye.
14) Elephant In The Room-a problem that everyone knows very well but no one talks about because it is taboo, embarrassing, etc-
The fact that his 35 year old brother was still financially dependent on his parents was a big elephant in the room at every family gathering.
15) Cast In The Same Mould-to be very similar-
Both the brothers are equally mean, they seem to be cast in the same mould.
16) Cross A Bridge Before One Comes To It-worry about the future in advance-
He was worried about his life after college, but then decided he will cross that bridge once he comes it.
17) Back To Square One-back to the beginning, return to where one started from-
The opposing parties had been trying to come to a compromise, but when the negotiations broke down, they were back to square one.
18) Back The Wrong Horse-make a wrong or inappropriate choice-
Only after investing all their money in the company did they realized that they had backed the wrong horse.
19) Let The Grass Grow Under Your Feet-to do nothing, delay-
We can't let the grass grow under our feet, we have got to get going with this project.
20) Rock The Boat-say or do something to disturb an existing situation and upset people-
After the recent riots, the government decided not to rock the boat by introducing new policies.