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prone (adj.)

c. 1400, "naturally inclined (to have or do something), apt, liable by disposition or tendency," from Latin pronus "bent forward, leaning forward, bent over," figuratively "inclined to, disposed," perhaps from adverbial form of pro "before, for, instead of" (see pro-) + ending as in infernus, externus.

The meaning "bending forward with the face down" is from 1570s; according to OED, the broader sense of "lying flat, in a horizontal position" (1690s) is "Permissible of things that have not an upper and under side, but improper of men and animals, unless the position is as in I" ["situated or lying face downward"]. Related: Proneness.

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Definitions of prone from WordNet

prone (adj.)
having a tendency (to); often used in combination;
failure-prone
a child prone to mischief
prone (adj.)
lying face downward;
Synonyms: prostrate
From wordnet.princeton.edu