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. 1600, from Late Latin susceptibilis "capable, sustainable, susceptible," from Latin suscept-, past-participle stem of suscipere "to take, catch, take up, lift up; receive, admit; submit to; sustain, support, bear; acknowledge, accept," from sub "up from under" (see sub-) + capere "to take," from PIE root *kap- "to grasp." Susceptive in the same sense is recorded from early 15c. Related: Susceptibly.
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<a href="https://www.etymonline.com/word/insusceptible">Etymology of insusceptible by etymonline</a>
Harper, D. (n.d.). Etymology of insusceptible. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/insusceptible