indeclinable adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

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.