Merriam-Webster Collegiate Dictionary - prone

 adjective  Etymology: Middle English, from Latin pronus bent forward, tending; akin to Latin pro forward — more at for  Date: 14th century  1. having a tendency or inclination ; being likely ~ to forget names accident-~  2.  a. having the front or ventral surface downward  b. lying flat or prostrate  • ~ adverb  • ~ly adverb  • ~ness noun Synonyms:  ~, supine, prostrate, recumbent mean lying down. ~ implies a position with the front of the body turned toward the supporting surface push-ups require a ~ position. supine implies lying on one's back and suggests inertness or abjectness lying supine on the couch. prostrate implies lying full-length as in submission, defeat, or physical collapse a runner fell prostrate at the finish line. recumbent implies the posture of one sleeping or resting a patient comfortably recumbent in a hospital bed.  Synonym: see in addition liable.