This has two meanings. Literally, it's floating, like a log in a river. But 99% of the time, it means your brain has left the chat. You're physically present in the meeting, but mentally, you're planning your dinner or replaying a K-drama scene. When someone asks you a question and all you can say is 'Ha?', you are officially 'lutang'. It's the default state of mind during a 3 PM meeting after a heavy lunch.