We have a few ways in English to talk about being crazy. Today’s idiom, to be “out of one’s mind” is one of them. So basically it means that someone is crazy. However, we don’t usually use this idiom when talking about a person with a serious mental illness; we usually use it when we are shocked by someone’s unusual behavior or idea. For example:
You want me to go bungee jumping with you? Are you out of your mind?!
I must have been out of my mind when I agreed to make dinner for ten people! It’s so much work!
Matt is out of his mind if he thinks he’ll be able to get into Harvard. He didn’t get very good grades in high school.
My friends like to get up at 5:30 a.m. and go jogging. They enjoy doing it but, personally, I think they’re out of their minds.
This expression can be a little strong when used directly with someone. So saying, “Are you out of your mind?” is quite strong. It’s ok to say this with close friends, but please be careful with your intonation. If your voice is very flat or if your intonation goes down, it will sound very negative. However, if your intonation is up and light, it will sound like a joke.
English Help Online's Blog 