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.
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.
1938, in biology, "a graded series of differences within a species," a back-formation from incline or from a Latinized form of Greek klinein "to slope, to lean" (from PIE root *klei- "to lean"). Earlier, but now obsolete, was a verb cline, from Middle English clinen "to bend, bow," from Old French cliner, from Latin clinare.
Proto-Indo-European root meaning "to lean."
It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").