I hope you remember that you shouldn't understand idioms literally, because if you try to read this one word by word, you will probably think that someone takes another person's arm and turns it around. It can be really painful, I guess. However, "twist someone's arm" idiom has nothing to do with pain. If your arm is twisted, it means that someone is doing a good job of persuading you to do something you might not have wanted to do.
And if you twist someone else’s arm, it means that you’re great at convincing them, and they finally agree to do something after you've been trying hard to talk talk them into it.
- Mike, you should really go to the movies with us tonight!
- You know I can’t, I have to do tons of homework.
- You know that Kelsey is going to be there, too, right?
- Kelsey? You've twisted my arm! Of course, I'll come!