late 14c., "to turn aside, deviate" (a sense now archaic), also "sink to a lower level," and, figuratively, "fall to an inferior or impaired condition," from Old French decliner "to sink, decline, degenerate, turn aside," from Latin 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."
In grammar, "to inflect as a noun or adjective," from late 14c. The sense has been altered by interpretation of de- as "downward;" intransitive meaning "to bend or slant down" is from c. 1400. Sense of "not to consent, politely refuse or withhold consent to do" is from 1630s. Related: Declined; declining.