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., deliberacioun, "act of weighing and examining," from Old French deliberation, from Latin deliberationem (nominative deliberatio), noun of action from past-participle stem of deliberare "consider carefully, consult," literally "weigh well," from de, here probably "entirely" (see de-) + -liberare, altered (probably by influence of liberare "to free, liberate") from librare "to balance, make level," from libra "pair of scales, a balance" (see Libra).
Sense of "slowness in decision or action" is from early 15c. Meaning "mutual discussion and examination of the reasons for and against a measure" is from late 15c..