1560s, "disposition to favor," with -ty + obsolete adjective propense "inclined, prone" (1520s), from Latin propensus, past participle of propendere "incline to, hang forward, hang down, weigh over," from pro "forward" (see pro-) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").
