Lacking the strength or ability to get up. Often used when someone is sick or injured, but it can be used figuratively as well. Oh man, I'm flat on my back with this flu. I haven't been out of bed in days! Sally's been flat on her back ever since she broke her leg. She's having a really hard time getting around in her cast. Losing my job has left me flat on my back. I'm really worried about how I'll pay my bills right now.