"unfinished, incomplete," 1640s, from Late Latin inconsummatus "unfinished," from in- "not" (see in- (1) + consummatus "perfected, complete," past participle of consummare "sum up, to complete" (see consummation).
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.
Middle English consummacioun, late 14c., "perfection;" c. 1400, "completion, accomplishment," from Latin consummationem (nominative consummatio) "a summing up; a finishing, a completing," noun of action from past-participle stem of consummare "to sum up, finish," from assimilated form of com "together, with" (see con-) + summa "sum, total," from summus "highest" (see sum (n.)). Sense of "completion of a marriage (by sexual intercourse)" is c. 1530.
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Harper, D. (n.d.). Etymology of inconsummate. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/inconsummate