incommensurate (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.

1640s, "corresponding in amount, degree, or magnitude," also "of equal size" (on the notion of "having the same boundaries"), from Late Latin commensuratus, from Latin com "with, together" (see com-) + Late Latin mensuratus, past participle of mensurare "to measure," from Latin mensura "a measuring, a measurement; thing to measure by," from mensus, past participle of metiri "to measure," from PIE root *me- (2) "to measure." Meaning "reducible to a common measure, commensurable" is from 1680s. Related: Commensurately.