in early use also encline, c. 1300, "to bend or bow toward," from Old French encliner "to lean, bend, bow down," from Latin inclinare "to cause to lean; bend, incline, turn, divert," from in- "into, in, on, upon" (from PIE root *en "in") + clinare "to bend" (from PIE *klein-, suffixed form of root *klei- "to lean"). Metaphoric sense of "have a mental disposition toward" is early 15c. in English (but existed in classical Latin). Related: Inclined; inclining.
c. 1600, "mental tendency," from incline (v.). The literal meaning "slant, slope" is attested from 1846 in railroading.