mid-14c., "lying face-down" (in submission, 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 from 1590s. General sense of "laid out, knocked flat" is from 1670s.
