A group of words established by usage as having a meaning not deducible from those of the individual words.

1. It is raining Cats and Dogs meaning it is raining heavily.

Example: I can’t make it on time for the meeting as it’s raining cats and dogs.

Do Pigs Fly? No. It’s important right? So is the meaning.

2. When Pigs Fly – That is impossible or never happens.

The phrase is often used for humorous effect, to scoff at over-ambition. There are numerous variations on the theme; when an individual with a reputation for failure finally succeeds, onlookers may sarcastically claim to see a flying pig.

3. Once in a blue moon – very rarely.

A blue moon is the term commonly used for a second full moon that occasionally appears in a single month of our solar-based calendars. In such a case, one of the four full moons in that season was labeled “blue.”

4. Break a leg – Good Luck!

This is an expression used mostly in the world of theatre to mean ‘Good Luck’ Actors and musicians are never wished ‘good luck’; before they walk on to the stage, they are usually told ‘break a leg’. This form of wishing people is beginning to be used in other contexts as well.

Example: I know you can do it. Break a leg!

5. Hit the books – to study with serious efforts.

Informal way if saying – to begin to study in a serious and determined way: I have to hit the books this weekend — I have two exams next week.

6. Digging your own grave – Doing something that will cause problems.

If you say that someone is digging their own grave, you are warning them that they are doing something foolish or dangerous that will cause their own failure. When someone takes unnecessary risks, like digging his own grave knowing he will get caught will be called as Daredevil.

Example: He dug his own grave knowing the consequences. You know, he is like that – a daredevil always.

7. Pull someone’s leg – kidding or joking.

If you accuse someone of pulling your leg, you mean that you believe they are teasing you. Usually you can tell if someone’s pulling your leg. Often the thing they are saying is difficult to believe or unrealistic. They might also be smiling when they say it. If you think someone is pulling your leg, say “haha pull the other one!”

8. Breaking new ground – Trying something new.

To do something innovative that is regarded as an advance or of positive benefit.Trying something new helps us break the routine. If we maintain a habit of trying new things, we progress and learn to be more open—open to change, new experiences, and to new possibilities.

9. Digging around – you try to find information about someone or something. 

Example: They said, after digging around, that the document was fake.

10. Take it with a pinch of salt – Don’t take it seriously.

So the truth is almost certainly that “with a grain (or pinch) of salt” originated sometime in the Middle Ages and always simply referred to making dull food more exciting (or a tall tale easier to swallow) by sprinkling a bit of salt on it. Later on, used to take things lightly.

Hope this helps. Thank you!