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decline: decline: [14] The notion underlying decline is of ‘bending away’. It comes via Old French decliner from Latin dēclināre ‘turn aside, go down’, a compound verb formed from the prefix dē-, ‘away, aside’ and clināre ‘bend’, which also produced English incline and recline and is related to lean. Its Latin nominal derivative dēclinātiō has bifurcated in English, to produce declination [14] and, via Old French declinaison, declension [15].

The latter is used only in the specialized grammatical sense ‘set of inflectional endings of a noun’, already present in Latin, which derives from the concept that every inflected form of a word represents a ‘falling away’ from its uninflected base form (the same underlying notion appears in the term oblique case ‘any grammatical sense other than the nominative or vocative’, and indeed the word case itself, whose etymological meaning is ‘fall’; and there are perhaps traces of it in inflection, literally ‘bending’).
=> declension, incline, lean, recline
declination (n.): late 14c. as a term in astronomy, from Old French declinacion (Modern French déclinaison), from Latin declinationem (nominative declinatio), noun of action from past participle stem of declinare (see decline (v.)). It took on various other senses 15c.-17c., most now obsolete.
decline (v.): late 14c., "to turn aside, deviate," from Old French decliner "to sink, decline, degenerate, turn aside," from Latin declinare "to lower, avoid, deviate, to bend from, inflect," from de- "from" (see de-) + clinare "to bend," from PIE *klei-n-, suffixed form of *klei- "to lean" (see lean (v.)). Sense has been altered since c. 1400 by interpretation of de- as "downward." Meaning "not to consent, politely refuse," is from 1630s. Related: Declined; declining.
decline (n.): early 14c., "deterioration, degeneration," from Old French declin (see decline (v.)).
declivity (n.): 1610s, from French déclivité, from Latin declivitatem (nominative declivitas) "a slope, declivity," from declivis "a sloping downward," from de- "down" (see de-) + clivus "a slope," from PIE *klei-wo-, suffixed form of *klei "to lean" (see lean (v.)).
indeclinable (adj.): late 14c., originally in grammar, from French indéclinable, from Latin indeclinabilis, from indeclinatus "unchanged, constant," from in- "not, opposite of" (see in- (1)) + declinatus, from declinare (see decline (v.)). Related: Indeclinably.