What does the idiom You must have been under a rock mean?

The idiom "You must have been under a rock" means you have been completely out of touch with what's been happening. It suggests that the person has been so unaware of current events, news, or popular culture that they are oblivious to something that is generally well-known.

Example:

"Did you not hear about the big concert last night? You must have been under a rock!"

Here, the speaker is implying that the person they are talking to should have known about the concert because it was a major event, and they are surprised that the person was unaware of it.

Other phrases that convey a similar meaning:

* Living under a rock

* Living in a cave

* Out of the loop

* Clueless

* Oblivious