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 15c. (Caxton), "easily taught, quick to learn," from Italian or French docile, from Latin docilis "easily taught," from docere "to show, teach, cause to know," originally "make to appear right," causative of decere "be seemly, fitting," from PIE root *dek- "to take, accept." Sense of "obedient, submissive, easily managed, tractable" is recorded by 1774. Middle English also had docible "ready or willing to teach" (c. 1400).
<a href="https://www.etymonline.com/word/indocile">Etymology of indocile by etymonline</a>
Harper, D. (n.d.). Etymology of indocile. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/indocile