What's the meaning of "check out"?

This is a comment made by a poster on an article about texting:

It seems to me that by texting during class or at meetings, you are in no position to assess whether the topic under discussion is worth your time and attention or not. You're not paying attention. You've already mentally checked out.

What does checked out mentioned above mean? I've looked it up in the dictionaries, but wasn't able to find a definition that's relevant to the context.

Answer

Literally it refers to "checking out of a hotel." It's something you do when you leave the hotel. Once you've checked out, you're gone.

This can be used figuratively in a few ways. The basic metaphor is that once you've checked out, you're gone. It could mean several things:

1. 
   

   Death. It's what you do when you leave this mortal life behind. Once you've checked out, you're gone.

   
   


2. 
   

   Senility. If you're still here physically but mentally you've checked out, then your body is still here but your mind is gone.

   
   

In your example, it's like #2 but perhaps without the sense of permanence. Physically speaking, you're sitting in class or you're at a meeting, but mentally speaking you're not there at all. It doesn't appear to suggest actual senility in this case, though—it's just being used to describe a lack of attention. This seems to me like a slightly more extended use of the metaphor.