c.1300, "to bend or bow toward," from O.Fr. encliner, from L. inclinare "to cause to lean," from in- "in" + clinare "to bend," from PIE *klei-n-, suffixed form of *klei "to lean" (see lean (v.)). Metaphoric sense of "have a mental disposition toward" is early 15c. in English (but existed in classical Latin). The noun meaning "slant, slope" is attested from 1846.