prone / prōn/ • adj. 1. (prone to/prone to do something) likely to or liable to suffer from, do, or experience something, typically something regrettable or unwelcome: years of logging had left the mountains prone to mudslides he is prone to jump to conclusions.2. lying flat, esp. face downward: I was lying prone on a foam mattress a prone position. ∎  technical denoting the position of the forearm with the palm of the hand facing downward.3. archaic with a downward slope or direction.DERIVATIVES: prone·ness n.

prone naturally inclined, disposed XIV; bending forward and downward XVI; lying flat XVII. — L. prōnus, f. prō forward (see PRO-1).
So pronation (physiol.) putting a fore limb into a prone position. XVII. — F. or medL. (f. late L. prōnāre). pronator muscle effecting this. XVIII.

prone (prohn) adj.
1. lying with the face downwards.

2. (of the forearm) in the position in which the palm of the hand faces downwards. Compare supine.