It is to be a person who lives in isolation from and has limited knowledge of what is happening in the world around them. Used to describe an ignorant or obtuse person. Usually someone who lives in the basement of their mothers house.
Someone who spends too much time playing Farmville on Facebook.
Often someone glued to their television or computer screen.
Usually brought up when someone makes a comment about a big movie that they have recently seen where the other person is oblivious to said movie.
Someone who lives in their basement and only comes out for special occasions.
"You've never heard of Obama? Where have you been for the last couple of years... living under a rock?"
"Unless you've been living under a rock, you'd know about..."
"You need to get out from under that rock you've been living under!"
"Wait... you don't know about that? Are you stupid!?"
"No, I have just been living under a rock recently."
"Who is our president"
"Are you serious!? Do you live under a rock?