Prone"ness, n.
1.
The quality or state of being prone, or of bending downward; as, the proneness of beasts is opposed to the erectness of man.
2.
The state of lying with the face down; -- opposed to supineness.
3.
Descent; declivity; as, the proneness of a hill.
4.
Inclination of mind, heart, or temper; propension; disposition; as, proneness to self-gratification.
© Webster 1913.