Why is the ninth month called September?
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.
early 14c., "deterioration, degeneration," from Old French declin (see decline (v.)).