The phrase “flat on one’s back” means: Helpless; without further recourse; at the end of one’s rope.
Although usually employed in a figurative sense, the original meaning was literal.
One who is actually flat on his back is so completely disabled through sickness or injury as to be helpless, unable to fend for himself.
Figuratively, the meaning may indicate helplessness through lack of power or through lack of financial means.
