"that cannot be discovered by searching, mysterious," c. 1500, from Late Latin inscrutabilis, from in- "not, opposite of" (see in- (1)) + *scrutabilis, from scrutari "examine, ransack" (see scrutiny). Related: Inscrutably; inscrutableness.
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.
early 15c., "a vote to choose someone to decide a question," from Late Latin scrutinium "a search, inquiry" (in Medieval Latin, "a mode of election by ballot"), from Latin scrutari "to examine, investigate, search," from PIE root *skreu- "to cut; cutting tool" (see shred (n.)). Meaning "close examination" first recorded c. 1600. Perhaps the original notion of the Latin word is "to search through trash," via scruta (plural) "trash, rags" ("shreds"); or the original sense might be "to cut into, scratch."