More phrases related to the body:

Don’t bite my head off!
When someone shouts at you or is annoyed with you, you can say “don’t bite my head off” and it basically means the same as “calm down” and that you want the person to stop being angry e.g.

Woman: You forgot to buy bread!
Man: OK, I’m sorry. Don’t bite my head off!

In the insect world, the female mantis is known for biting off the male’s head after copulation (sex), though I’m not sure if this is the origin of this particular idiom… maybe it is!

By heart
If you know something by heart, you know it very well. According to Callan, the best way to learn is to repeat, repeat, and repeat, so that the learner can speak without thinking. The student, therefore, knows the answers to the questions by heart. Most people know the alphabet by heart, unless you live in China where there is no alphabet.

Chin wag
The chin is part of the face, below the mouth, and to wag means to move from side to side, like a dog’s tail. So, a chin wag is a chat or an informal conversation e.g. Let’s go to the pub for a beer and a chin wag.