irresistance n.

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.

mid-14c., resistence, "moral or political opposition;" late 14c., "military or armed physical opposition by force; difficulty, trouble," from Old French resistance, earlier resistence, and directly from Medieval Latin resistentia, from present-participle stem of Latin resistere "make a stand against, oppose" (see resist).

From 1580s as "power or capacity of resisting." The meaning "organized covert opposition to an occupying or ruling power" [OED] is from 1939. The electromagnetic sense of "non-conductivity" is from 1760. Also used  in science and engineering with a sense of "force exerted by a medium to retard motion through it," hence the figurative phrase path of least resistance "easiest method or course" (1825), earlier a term in physical sciences and engineering.