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.
c. 1300, "ready and able to fight, able to defend," from Old French defensable, from Medieval Latin defensibilis, Late Latin defensibilem, from Latin defens-, past-participle stem of defendere (see defend). Meaning "capable of being defended" is from late 14c., sense of "contributing to defense" is from c. 1400; that of "that may be vindicated" is from early 15c. Related: Defensibility.
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Harper, D. (n.d.). Etymology of indefensible. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/indefensible