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.
1570s, "consisting" (a sense now obsolete), from Latin consistentem (nominative consistens), present participle of consistere "to stand firm, take a standing position, stop, halt," from assimilated form of com "with, together" (see con-) + sistere "to place," causative of stare "to stand, be standing," from PIE root *sta- "to stand, make or be firm."
Sense of "standing together in agreement, agreeing" (with with) is first attested 1640s; meaning "marked by consistency" is from 1732. The literal, physical sense survives in consistency. Related: Consistently.
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Harper, D. (n.d.). Etymology of inconsistent. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/inconsistent