Living Under a Rock

This is quite a useful phrase in spoken English. If you are “living under a rock” it means
that you haven’t been paying attention to what is going on in the world around you.  It’s almost as if you have been hiding from all the news.

If you didn’t know that Obama was the US president, someone could ask you, “How is it possible you didn’t know that?  Have you been living under a rock?”

Another good example would be if your friend hadn’t heard a really popular song they always play in the bars, clubs, and in popular movies.

You could say, “I can’t believe you haven’t heard this song before.  You must have been living under a rock the past year!“