Definition of "prone"

prone

/pɹəʊn/

adjective
comparative proner or more prone, superlative pronest or most prone
1. Lying face downward.
  Quotations
  But they had already discovered that he could be bullied, and they had it their own way; and presently Selwyn lay prone upon the nursery floor, impersonating a ladrone while pleasant shivers chased themselves over Drina, whom he was stalking.
  1907 August, Robert W[illiam] Chambers, chapter I, in The Younger Set, New York, N.Y.: D. Appleton & Company
2. Of the hand, forearm or foot: turned facing away from the body; with the thumb inward or big toe downward.
3. Having a downward inclination or slope.
4. (figuratively) Predisposed, liable, inclined.
  Quotations
  Nor are mine ears with thy tongue's tune delighted; / Nor tender feeling, to base touches prone, / Nor taste, nor smell, desire to be invited / To any sensual feast with thee alone: […]
  1609, William Shakespeare, “Sonnet 141”, in Shake-speares Sonnets. […]‎, London: By G[eorge] Eld for T[homas] T[horpe] and are to be sold by William Aspley
verb
third-person singular simple present prones, present participle proning, simple past and past participle proned
1. (medicine) To place in a prone position, to place face down.