"not admitting of dispute or debate, too clear to be controverted," 1670s, from in- "not" (see in- (1)) + contestable (see contest (v.)). Perhaps from or modeled on French incontestable. Related: Incontestably.
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, "fight or do battle for, strive to win or hold," from French contester "dispute, oppose," from Latin contestari (litem) "to call to witness, bring action," from assimilated form of com "with, together" (see con-) + testari "to bear witness," from testis "a witness," (see testament).
The notion of the Latin compound is "calling witnesses" as the first step in a legal combat. Meaning "make a subject of contention or dispute, enter into competition for" is from 1610s. Sense of "to argue in opposition, call into question" is from 1660s. Related: Contestable; contested; contesting.
<a href="https://www.etymonline.com/word/incontestable">Etymology of incontestable by etymonline</a>
Harper, D. (n.d.). Etymology of incontestable. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/incontestable