Origin and history of inarticulate

early 15c., articulacioun (Chauliac), "a joint or joining; setting of bones," from Old French articulation, from Medieval Latin articulationem (nominative articulatio) "separation into joints," noun of action from past-participle stem of articulare "to separate (meat) into joints," also "to utter distinctly," from articulus, diminutive of artus "joint" (see article). The meaning "the uttering of articulate sounds" is from 1610s.

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.