proclivity (n.)

1590s, from Middle French proclivité or directly from Latin proclivitatem (nominative proclivitas) "a tendency, predisposition, propensity," from proclivis "prone to," literally "sloping, inclined," from pro "forward" (see pro-) + clivus "a slope," from PIE *klei-wo-, suffixed form of root *klei- "to lean."