To mentally clock off and stop paying attention, even though you're still sat there nodding like a plastic dog in a car window. Your eyes are on the speaker, your brain is off on a side quest. Classic in long meetings, dull lessons, or when someone starts telling you their fantasy football stats. Honestly, happens to the best of us.
"Sorry mate, I proper zoned out, what did you say? I said it’s your round. Oh no, I’ve been spiritually absent and now I’m skint at the bar"