late 14c., inclinacioun, "condition of being mentally disposed" (to do something), "natural disposition due to a humor or the influence of planets at one's birth," from Old French inclination (14c.) and directly from Latin inclinationem (nominative inclinatio) "a leaning, bending," figuratively "tendency, bias, favor," noun of action from past-participle stem of inclinare "to bend, turn; cause to lean" (see incline (v.)). Meaning "action of bending toward" (something) is from early 15c. That of "amount of a slope" is from 1799.
updated on November 23, 2020