Idiomatic expressions are a type of informal language that have a meaning different from the meaning of the words in the expression.
This idiom doesn't actually mean that you should stick your fingers in your mouth and grab a hold of your tongue. It means that you shouldn't talk.
People "hold their tongues" when they're in situations where they want to talk, but it would be better if they didn't. So, while their tongue is ready to do some talking, they "hold" it and don't say anything.
1. She was tickled pink by the good news. ---> made very happy
2. You are hands down the best player on the team. ---> There was no competition.
3. He's been down in the dumps lately. ---> sad or depressed
4. I feel sick as a dog. ---> very sick
5. My grandma has been under the weather. ---> not well
6. Rise and shine! ---> Wake up and be happy!
7. Close, but no cigar. ---> You were very close, but you did not make it.
8. I could play outside till the cows come home. ---> for a very long time
9. Bring your umbrella. It's raining cats and dogs out there! ---> very hard rain
10. That sound is driving me up the wall! ---> making me very annoyed
11. This assignment is a piece of cake. ---> very easy
12. Although he broke a serious rule, he was only given a slap on the wrist. ---> A mild punishment
13. Yikes! This shirt costs an arm and a leg. ---> It is extremely expensive.
14. I was just pulling your leg. ---> just joking
15. It's Greek to me. ---> I don't understand.
16. Keep your chin up. ---> Be happy.
17. Hold your horses. ---> Be patient.
18. We're all in the same boat. ---> All of us are in the same position.
19. I will clean my room when pigs fly. ---> never
20. He's a loose cannon.* ---> unpredictable
* As Wikipedia describes it, the literal definition of a loose cannon is, "a cannon which gets dislocated and moves about randomly on the decks of a battleship, creating a hazard to crew and equipment." This never meant much to me until I read Victor Hugo's novel "Ninety-Three." You can read the beautiful description here if you'd like. :)