mid-14c., "lying face-down, at length on the ground" (in submission, supplication, humility, worship, etc.), from Latin prostratus, past participle of prosternere "strew in front, throw down," from pro "before, forth" (see pro-) + sternere "to spread out, lay down, stretch out" (from nasalized form of PIE root *stere- "to spread"). Figurative use is from 1590s. General sense of "laid out, knocked flat" is from 1670s.
early 15c., prostraten, "prostrate oneself, fall down flat, bow with the face to the ground" (in humility or submission), from prostrate (adj.). Transitive sense of "throw down, lay flat, overthrow" is by 1560s. Related: Prostrated; prostrating.