Darme el avión

Darme el avión means to humor you, to pretend to be listening to what you are saying. Here are some examples:

• No me des el avión. Don't ignore me when I'm speaking!
• Me estás dando el avionazo. You're not even listening to me!

This is the perfect expression for when someone is nodding along with you but in reality they have stopped paying attention. It also works for when they are pretending to be taking your concerns seriously when in fact they don't. Imagine a customer service representative telling you that they will look into the problem, when in reality they are just saying that to get you off the phone.

Se me fue el avión

You can use se me fue el avión, the plane left without me, when the distraction interferes not with your listening but with your own actions.

• Se me fue el avión. Sorry, I lost my train of thought. What was I saying?
• Ay, qué mensa (soy). Se me fue el avión. Oops, I forgot to do it. Se me olvidó.
• Se me fue el avión y no te llamé. I was going to call you but it slipped my mind.

As you can see, se me fue el avión works not just for forgetting what you were going to say in the moment but for also for forgetting to do something in general.

Estar en la luna

The expression estar/andar en la luna works for general distraction:

• No, perdóname, es que estaba en la luna. Sorry, I got so distracted that I stopped listening.
• Estás en la luna. You seem kind of distracted.
• Te la vives en el luna. You are distracted in general.

I've also heard estar en la baba, meaning be spaced out, literally to be drooling.