prone 
(prōn)
(prōn)adj.
1. Lying with the front or face downward.
2. Having a tendency; inclined. Often used in combination: paper that is prone to yellowing; an accident-prone child.
adv.
In a prone manner: The patient was lying prone on the bed.
[Middle English, inclined, disposed, from Latin prōnus, leaning forward; see per1 in the Appendix of Indo-European roots.]
pronely adv.
proneness n.