To have no 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 on her back ever since she broke her leg. She's having a really hard time getting around in her cast. I've been flat on my back since I lost my job. I'm really worried about how I'll pay my bills right now.