An Etymological Dictionary of Astronomy and Astrophysics

1a) To bend down; slope downward; descend.

1b) Grammar: To → inflect according to the various cases of a noun, adjective, or pronoun.

2. The process of declining; a downward movement; gradual loss or deterioration.

Etymology (EN): M.E. declinen, from O.Fr. decliner “to sink, decline, degenerate, turn aside,” from declinare
“to turn away, inflect,” from → de- + clinare “to bend,” cf. Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” Mod.Pers. kil “bent, inclined, slant,” PIE base *klei- “to lean, incline” from which is also derived
(Ger. lehnen, E. lean).

Etymology (PE): Vâkil, from prefix vâ- “away” + kil “bent, crooked, inclined” [Dehxodâ], Mid.Pers. xwahl “bent, crooked” (Mod.Pers. dialectal/colloquial variants kowleh, cowleh, cowl, caval, xohl, xohal, hol), cf. Skt. kûla- “slope, declivity;” PIE base *klei-, as above. → inclination