Quiz: Remember the definition of mal de mer?
c.1400, "naturally inclined to something, apt, liable," 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. Meaning "lying face-down" is first recorded 1570s. Literal and figurative senses both were in Latin; figurative is older in English. Related: Proneness.
Lying with the front or face downward.
Having a tendency; inclined.