Zone Out

Idiom: to zone out
Example 1:

Tim: How was your history class today?

Chris: Well, I didn't sleep well last night, so I was so tired.  I pretty much zoned out the whole class.

Tim: You still look a little sleepy.  You'd better get some good sleep tonight.


Example 2:

Sara: Hey, Jill! Focus on the road!

Jill: I'm sorry. I was zoning out.

Sara: What were you thinking about? 

Jill: My boyfriend and I have been having problems and I guess I just started daydreaming.


Meaning: to zone outmeans that someone cannot concentrate and their mind starts to wander or daydream.