"Flat on one's back" is a phrase used to describe a person who is lying down on their back, either due to illness, injury, or exhaustion. There are a variety of synonyms that can be used to describe this position, including reclined, horizontal, supine, prostrate, recumbent, prone, or lying down. Each of these synonyms describes a slightly different angle or position of the body, but all convey the idea of a person lying flat on their back. Using these synonyms can make writing more interesting and varied, and can help to avoid repetitive phrasing when describing a person's position.