declension (n.)

mid-15c., declinson, in grammar, "the inflection of nouns, pronouns, and adjectives, especially with a change in form from the nominative case," ultimately from Latin declinationem (nominative declinatio) "a bending from (something), a bending aside; a turning away from (something), an avoiding," also used in the grammatical sense, noun of action from past-participle stem of declinare "to lower; avoid, deviate; bend from, inflect," from de "from" (see de-) + clinare "to bend," from PIE *klein-, suffixed form of root *klei- "to lean."

The immediate source of the English word is perhaps in French (compare Old French declinaison), but "the form is irregular, and its history obscure" [OED]. Meaning "a sloping downward" is from 1640s; that of "a sinking or falling into a lower or inferior state" is from c. 1600; that of "courteous refusal, non-acceptance" is by 1817. Related: Declensional.